diff --git a/app/(tools)/cron-explainer/CronExplainerClient.tsx b/app/(tools)/cron-explainer/CronExplainerClient.tsx
index 5fe7079..9f28487 100644
--- a/app/(tools)/cron-explainer/CronExplainerClient.tsx
+++ b/app/(tools)/cron-explainer/CronExplainerClient.tsx
@@ -124,7 +124,7 @@ export function CronExplainerClient() {
         </div>
 
         {!parsed.ok && expression.trim() !== "" && (
-          <p role="alert" className="rounded-xl border border-[var(--rose)]/30 bg-[var(--rose)]/5 p-3 text-sm text-[var(--rose)]">
+          <p role="alert" className="rounded-xl border border-tint-rose-30 bg-tint-rose-5 p-3 text-sm text-[var(--rose)]">
             {parsed.error}
           </p>
         )}
diff --git a/app/(tools)/csv-formatter/components/IssueList.tsx b/app/(tools)/csv-formatter/components/IssueList.tsx
index 203c127..5d8674b 100644
--- a/app/(tools)/csv-formatter/components/IssueList.tsx
+++ b/app/(tools)/csv-formatter/components/IssueList.tsx
@@ -10,7 +10,7 @@ export function IssueList({ issues }: { issues: CsvIssue[] }) {
     return (
       <p
         role="status"
-        className="flex items-center gap-2 rounded-xl border border-[var(--green)]/30 bg-[var(--green)]/5 p-3 text-sm text-[var(--green)]"
+        className="flex items-center gap-2 rounded-xl border border-tint-green-30 bg-tint-green-5 p-3 text-sm text-[var(--green)]"
       >
         <CheckCircle2 className="h-4 w-4 shrink-0" aria-hidden="true" />
         No problems found — every row matches the header.
diff --git a/app/(tools)/markdown-preview/MarkdownPreviewClient.tsx b/app/(tools)/markdown-preview/MarkdownPreviewClient.tsx
index 9f83451..fb11b17 100644
--- a/app/(tools)/markdown-preview/MarkdownPreviewClient.tsx
+++ b/app/(tools)/markdown-preview/MarkdownPreviewClient.tsx
@@ -113,7 +113,7 @@ export function MarkdownPreviewClient() {
       <button type="button" className="btn" onClick={downloadHtml} disabled={!markdown}>Download HTML</button>
       <button type="button" className="btn" onClick={download} disabled={!markdown}>Download .md</button>
     </div>
-    {importError && <p role="alert" className="rounded-md border border-[var(--rose)]/30 bg-[var(--rose)]/5 px-3 py-2 text-[13px] text-[var(--rose)]">{importError}</p>}
+    {importError && <p role="alert" className="rounded-md border border-tint-rose-30 bg-tint-rose-5 px-3 py-2 text-[13px] text-[var(--rose)]">{importError}</p>}
     <div className="grid grid-cols-1 lg:grid-cols-2 gap-3">
       <div className="card overflow-hidden"><div className="px-3 py-2 border-b border-[var(--hairline)] bg-[var(--bg-raise)] font-mono text-[10.5px] tracking-[0.22em] uppercase text-[var(--fg-dim)]">Markdown</div><textarea ref={textareaRef} aria-label="Markdown input" value={markdown} onChange={(e) => setMarkdown(e.target.value)} spellCheck={false} className="editor-input p-4" style={{ minHeight: 520 }} /></div>
       <div className="card overflow-hidden" style={{ background: darkPreview ? "#17191f" : "#ffffff", color: darkPreview ? "#f4f4f5" : "#17191f" }}><div className="px-3 py-2 border-b border-black/10 font-mono text-[10.5px] tracking-[0.22em] uppercase opacity-60">Preview</div><article className="markdown-preview p-5 min-h-[520px] overflow-auto" dangerouslySetInnerHTML={{ __html: html }} /></div>
diff --git a/app/(tools)/number-base-converter/NumberBaseConverterClient.tsx b/app/(tools)/number-base-converter/NumberBaseConverterClient.tsx
index 6c415a3..e7ebc33 100644
--- a/app/(tools)/number-base-converter/NumberBaseConverterClient.tsx
+++ b/app/(tools)/number-base-converter/NumberBaseConverterClient.tsx
@@ -183,7 +183,7 @@ export function NumberBaseConverterClient() {
           <p
             id="nb-error"
             role="alert"
-            className="rounded-xl border border-[var(--rose)]/30 bg-[var(--rose)]/5 p-3 text-sm text-[var(--rose)]"
+            className="rounded-xl border border-tint-rose-30 bg-tint-rose-5 p-3 text-sm text-[var(--rose)]"
           >
             {error}
           </p>
@@ -267,7 +267,7 @@ export function NumberBaseConverterClient() {
             </select>
           </div>
           {twos && "error" in twos ? (
-            <p role="alert" className="rounded-xl border border-[var(--rose)]/30 bg-[var(--rose)]/5 p-3 text-sm text-[var(--rose)]">
+            <p role="alert" className="rounded-xl border border-tint-rose-30 bg-tint-rose-5 p-3 text-sm text-[var(--rose)]">
               {twos.error}
             </p>
           ) : (
diff --git a/app/(tools)/regex-tester/RegexTesterClient.tsx b/app/(tools)/regex-tester/RegexTesterClient.tsx
index 0404a76..629df04 100644
--- a/app/(tools)/regex-tester/RegexTesterClient.tsx
+++ b/app/(tools)/regex-tester/RegexTesterClient.tsx
@@ -100,7 +100,7 @@ export function RegexTesterClient() {
         </div>
 
         {!compiled.ok && (
-          <p role="alert" className="rounded-xl border border-[var(--rose)]/30 bg-[var(--rose)]/5 p-3 text-sm text-[var(--rose)]">
+          <p role="alert" className="rounded-xl border border-tint-rose-30 bg-tint-rose-5 p-3 text-sm text-[var(--rose)]">
             {compiled.error}
           </p>
         )}
diff --git a/app/(tools)/timestamp-converter/TimestampConverterClient.tsx b/app/(tools)/timestamp-converter/TimestampConverterClient.tsx
index e2a9067..371de00 100644
--- a/app/(tools)/timestamp-converter/TimestampConverterClient.tsx
+++ b/app/(tools)/timestamp-converter/TimestampConverterClient.tsx
@@ -94,7 +94,7 @@ export function TimestampConverterClient() {
             </div>
           </div>
           {!epochResult.ok ? (
-            <p role="alert" className="rounded-xl border border-[var(--rose)]/30 bg-[var(--rose)]/5 p-3 text-sm text-[var(--rose)]">{epochResult.error}</p>
+            <p role="alert" className="rounded-xl border border-tint-rose-30 bg-tint-rose-5 p-3 text-sm text-[var(--rose)]">{epochResult.error}</p>
           ) : (
             <div className="space-y-3">
               <p className="text-xs text-[var(--fg-muted)]">Interpreted as {UNIT_OPTIONS.find((option) => option.value === epochResult.unitUsed)?.label}</p>
@@ -115,7 +115,7 @@ export function TimestampConverterClient() {
           <label className="block text-sm font-medium text-[var(--fg)]" htmlFor="date-input">Date or ISO 8601</label>
           <input id="date-input" value={dateInput} onChange={(e) => setDateInput(e.target.value)} spellCheck={false} placeholder="2024-02-29T00:00:00Z" className="w-full rounded-xl border border-[var(--hairline)] bg-[var(--bg)] px-3 py-3 font-mono text-sm text-[var(--fg)] outline-none focus:border-[var(--green)]" />
           {!dateResult.ok ? (
-            <p role="alert" className="rounded-xl border border-[var(--rose)]/30 bg-[var(--rose)]/5 p-3 text-sm text-[var(--rose)]">{dateResult.error}</p>
+            <p role="alert" className="rounded-xl border border-tint-rose-30 bg-tint-rose-5 p-3 text-sm text-[var(--rose)]">{dateResult.error}</p>
           ) : (
             <div className="space-y-3">
               <Field label="Unix seconds" value={String(dateResult.seconds)} copyValue={String(dateResult.seconds)} />
diff --git a/app/(tools)/uuid-generator/UuidGeneratorClient.tsx b/app/(tools)/uuid-generator/UuidGeneratorClient.tsx
index 5fd6a33..abf7d52 100644
--- a/app/(tools)/uuid-generator/UuidGeneratorClient.tsx
+++ b/app/(tools)/uuid-generator/UuidGeneratorClient.tsx
@@ -26,7 +26,7 @@ function compareUuidBytes(a: Uint8Array, b: Uint8Array): number {
 function InspectionResult({ result }: { result: UuidInspection }) {
   if (!result.ok) {
     return (
-      <p role="alert" className="rounded-xl border border-[var(--rose)]/30 bg-[var(--rose)]/5 p-3 text-sm text-[var(--rose)]">
+      <p role="alert" className="rounded-xl border border-tint-rose-30 bg-tint-rose-5 p-3 text-sm text-[var(--rose)]">
         Enter a valid UUID in hyphenated or compact format.
       </p>
     );
@@ -190,7 +190,7 @@ export function UuidGeneratorClient() {
         </div>
 
         {generationError && (
-          <p role="alert" className="rounded-xl border border-[var(--rose)]/30 bg-[var(--rose)]/5 p-3 text-sm text-[var(--rose)]">
+          <p role="alert" className="rounded-xl border border-tint-rose-30 bg-tint-rose-5 p-3 text-sm text-[var(--rose)]">
             {generationError}
           </p>
         )}
diff --git a/app/guides/convert-unix-timestamp-to-date/page.tsx b/app/guides/convert-unix-timestamp-to-date/page.tsx
index dc9c0ab..1262789 100644
--- a/app/guides/convert-unix-timestamp-to-date/page.tsx
+++ b/app/guides/convert-unix-timestamp-to-date/page.tsx
@@ -195,7 +195,7 @@ export default function ConvertUnixTimestampToDateGuidePage() {
                   className="relative p-6 rounded-lg border border-[var(--hairline)] bg-[var(--bg-raise)] scroll-mt-20"
                 >
                   <div className="flex items-center gap-3 mb-3">
-                    <span className="flex items-center justify-center w-7 h-7 rounded-full bg-[var(--green)]/10 border border-[var(--green)]/30 font-mono text-xs font-bold text-[var(--green)]">
+                    <span className="flex items-center justify-center w-7 h-7 rounded-full bg-tint-green-10 border border-tint-green-30 font-mono text-xs font-bold text-[var(--green)]">
                       {idx + 1}
                     </span>
                     <h3 className="font-display text-lg font-semibold text-[var(--fg)]">
@@ -328,7 +328,7 @@ dt = epoch + timedelta(seconds=-86400)
           </section>
 
           {/* Section: Year 2038 */}
-          <section className="p-6 rounded-lg border border-[var(--amber)]/40 bg-[var(--bg-raise)]">
+          <section className="p-6 rounded-lg border border-tint-amber-40 bg-[var(--bg-raise)]">
             <div className="flex items-center gap-2 font-mono text-xs font-semibold uppercase tracking-wider text-[var(--amber)] mb-3">
               <AlertTriangle className="w-4 h-4" />
               Year 2038 limit
diff --git a/app/guides/create-a-professional-invoice/page.tsx b/app/guides/create-a-professional-invoice/page.tsx
index cdf4445..1910872 100644
--- a/app/guides/create-a-professional-invoice/page.tsx
+++ b/app/guides/create-a-professional-invoice/page.tsx
@@ -219,7 +219,7 @@ export default function CreateProfessionalInvoiceGuidePage() {
                   className="relative p-6 rounded-lg border border-[var(--hairline)] bg-[var(--bg-raise)] scroll-mt-20"
                 >
                   <div className="flex items-center gap-3 mb-3">
-                    <span className="flex items-center justify-center w-7 h-7 rounded-full bg-[var(--green)]/10 border border-[var(--green)]/30 font-mono text-xs font-bold text-[var(--green)]">
+                    <span className="flex items-center justify-center w-7 h-7 rounded-full bg-tint-green-10 border border-tint-green-30 font-mono text-xs font-bold text-[var(--green)]">
                       {idx + 1}
                     </span>
                     <h3 className="font-display text-lg font-semibold text-[var(--fg)]">
diff --git a/app/guides/decode-jwt/page.tsx b/app/guides/decode-jwt/page.tsx
index d7452b6..3eb4dd9 100644
--- a/app/guides/decode-jwt/page.tsx
+++ b/app/guides/decode-jwt/page.tsx
@@ -190,7 +190,7 @@ export default function DecodeJwtGuidePage() {
                   className="relative p-6 rounded-lg border border-[var(--hairline)] bg-[var(--bg-raise)] scroll-mt-20"
                 >
                   <div className="flex items-center gap-3 mb-3">
-                    <span className="flex items-center justify-center w-7 h-7 rounded-full bg-[var(--violet)]/10 border border-[var(--violet)]/30 font-mono text-xs font-bold text-[var(--violet)]">
+                    <span className="flex items-center justify-center w-7 h-7 rounded-full bg-tint-violet-10 border border-tint-violet-30 font-mono text-xs font-bold text-[var(--violet)]">
                       {idx + 1}
                     </span>
                     <h3 className="font-display text-lg font-semibold text-[var(--fg)]">
@@ -355,7 +355,7 @@ console.log(decode(payloadPart));`}
             </div>
           </section>
 
-          <section className="p-6 rounded-lg border border-[var(--amber)]/40 bg-[var(--bg-raise)]">
+          <section className="p-6 rounded-lg border border-tint-amber-40 bg-[var(--bg-raise)]">
             <div className="flex items-center gap-2 font-mono text-xs font-semibold uppercase tracking-wider text-[var(--amber)] mb-3">
               <ShieldAlert className="w-4 h-4" /> Decoding is not verification
             </div>
diff --git a/app/guides/format-and-validate-json/page.tsx b/app/guides/format-and-validate-json/page.tsx
index 745ea73..eab31b0 100644
--- a/app/guides/format-and-validate-json/page.tsx
+++ b/app/guides/format-and-validate-json/page.tsx
@@ -165,7 +165,7 @@ export default function FormatAndValidateJsonGuidePage() {
                   className="relative p-6 rounded-lg border border-[var(--hairline)] bg-[var(--bg-raise)] scroll-mt-20"
                 >
                   <div className="flex items-center gap-3 mb-3">
-                    <span className="flex items-center justify-center w-7 h-7 rounded-full bg-[var(--cyan)]/10 border border-[var(--cyan)]/30 font-mono text-xs font-bold text-[var(--cyan)]">
+                    <span className="flex items-center justify-center w-7 h-7 rounded-full bg-tint-cyan-10 border border-tint-cyan-30 font-mono text-xs font-bold text-[var(--cyan)]">
                       {idx + 1}
                     </span>
                     <h3 className="font-display text-lg font-semibold text-[var(--fg)]">
diff --git a/app/guides/wcag-color-contrast-ratio/page.tsx b/app/guides/wcag-color-contrast-ratio/page.tsx
index 00cb732..0ca689d 100644
--- a/app/guides/wcag-color-contrast-ratio/page.tsx
+++ b/app/guides/wcag-color-contrast-ratio/page.tsx
@@ -281,7 +281,7 @@ const contrast = (a, b) => {
                   className="relative p-6 rounded-lg border border-[var(--hairline)] bg-[var(--bg-raise)] scroll-mt-20"
                 >
                   <div className="flex items-center gap-3 mb-3">
-                    <span className="flex items-center justify-center w-7 h-7 rounded-full bg-[var(--fuchsia)]/10 border border-[var(--fuchsia)]/30 font-mono text-xs font-bold text-[var(--fuchsia)]">
+                    <span className="flex items-center justify-center w-7 h-7 rounded-full bg-tint-fuchsia-10 border border-tint-fuchsia-30 font-mono text-xs font-bold text-[var(--fuchsia)]">
                       {idx + 1}
                     </span>
                     <h3 className="font-display text-lg font-semibold text-[var(--fg)]">
diff --git a/lib/__tests__/tint-classes.test.ts b/lib/__tests__/tint-classes.test.ts
new file mode 100644
index 0000000..fdb7280
--- /dev/null
+++ b/lib/__tests__/tint-classes.test.ts
@@ -0,0 +1,84 @@
+import { describe, expect, it } from "vitest";
+import { readdirSync, readFileSync, statSync } from "node:fs";
+import { join, relative } from "node:path";
+import postcss from "postcss";
+import tailwindcss from "tailwindcss";
+import config from "../../tailwind.config";
+
+// Tailwind 3 cannot put an opacity on a var() colour. `border-[var(--rose)]/30` builds no CSS at all, so the box
+// silently falls back to the default grey border with no tint. The tinted boxes use `border-tint-rose-30` and
+// `bg-tint-rose-5` instead (see tailwind.config.ts).
+
+const ROOT = join(__dirname, "..", "..");
+const SOURCE_DIRS = ["app", "components", "lib"];
+const SOURCE_FILE = /\.(tsx?|jsx?|mdx|css)$/;
+
+function sourceFiles(dir: string): string[] {
+  return readdirSync(dir).flatMap((name) => {
+    const path = join(dir, name);
+    if (statSync(path).isDirectory()) return name === "__tests__" || name === "node_modules" ? [] : sourceFiles(path);
+    return SOURCE_FILE.test(name) ? [path] : [];
+  });
+}
+
+const files = SOURCE_DIRS.flatMap((dir) => sourceFiles(join(ROOT, dir)));
+const text = (file: string) => readFileSync(file, "utf8");
+
+// A utility whose value is a var() colour followed by an opacity: `-[var(--x)]/30`, `-[var(--x)]/[0.3]`.
+const VAR_OPACITY = /-\[var\(--[\w-]+\)\]\/(\d|\[)/;
+const TINT_CLASS = /\b(?:bg|border|text|ring|outline|divide|fill|stroke|shadow|from|via|to)-tint-[a-z]+-\d+\b/g;
+
+describe("tinted boxes", () => {
+  it("finds the source files it checks", () => {
+    expect(files.length).toBeGreaterThan(50);
+  });
+
+  it("uses no [var(--x)]/NN opacity class, which Tailwind 3 never builds", () => {
+    const offenders = files.flatMap((file) =>
+      text(file)
+        .split("\n")
+        .flatMap((line, i) => (VAR_OPACITY.test(line) ? [`${relative(ROOT, file)}:${i + 1}`] : [])),
+    );
+    expect(offenders).toEqual([]);
+  });
+
+  it("catches the class it bans", () => {
+    for (const bad of ["border-[var(--rose)]/30", "bg-[var(--cyan)]/5", "text-[var(--fg-muted)]/[0.4]"]) {
+      expect(VAR_OPACITY.test(`className="p-3 ${bad}"`)).toBe(true);
+    }
+    for (const fine of ["border-[var(--rose)]", "bg-tint-rose-5", "border-tint-green-30", "w-1/2", "bg-accent-rose/30"]) {
+      expect(VAR_OPACITY.test(`className="p-3 ${fine}"`)).toBe(false);
+    }
+  });
+
+  describe("the tint classes", () => {
+    const used = [...new Set(files.flatMap((file) => text(file).match(TINT_CLASS) ?? []))].sort();
+
+    async function build(classes: string[]): Promise<string> {
+      const result = await postcss([
+        tailwindcss({ ...config, content: [{ raw: classes.join(" "), extension: "html" }] }),
+      ]).process("@tailwind utilities;", { from: undefined });
+      return result.css;
+    }
+
+    it("are used by the boxes that were missing their tint", () => {
+      expect(used.length).toBeGreaterThanOrEqual(10);
+      expect(used).toContain("border-tint-rose-30");
+      expect(used).toContain("bg-tint-rose-5");
+    });
+
+    it("are all built by Tailwind, each as a mix of the accent with transparent", async () => {
+      const css = await build(used);
+      for (const name of used) {
+        const [, accent, step] = /tint-([a-z]+)-(\d+)$/.exec(name) as RegExpExecArray;
+        const property = name.startsWith("bg-") ? "background-color" : "border-color";
+        expect(css, name).toContain(`.${name} {`);
+        expect(css, name).toContain(`${property}: color-mix(in srgb, var(--${accent}) ${step}%, transparent)`);
+      }
+    });
+
+    it("is not built for a percentage nobody added", async () => {
+      expect(await build(["border-tint-rose-35"])).not.toContain("tint-rose-35");
+    });
+  });
+});
diff --git a/tailwind.config.ts b/tailwind.config.ts
index f4eb09f..da85a23 100644
--- a/tailwind.config.ts
+++ b/tailwind.config.ts
@@ -1,5 +1,17 @@
 import type { Config } from "tailwindcss";
 
+// Tailwind 3 cannot put an opacity on a var() colour, so `border-[var(--rose)]/30` builds no CSS at all and the
+// box falls back to the default grey border with no tint. `border-tint-rose-30` and `bg-tint-rose-5` mix the
+// accent with transparent instead (the accent itself follows the theme). For another percentage, add it to TINT_STEPS.
+const TINT_ACCENTS = ["cyan", "amber", "rose", "violet", "sky", "lime", "green", "fuchsia"] as const;
+const TINT_STEPS = [5, 10, 30, 40] as const;
+const tint = Object.fromEntries(
+  TINT_ACCENTS.map((accent) => [
+    accent,
+    Object.fromEntries(TINT_STEPS.map((step) => [step, `color-mix(in srgb, var(--${accent}) ${step}%, transparent)`])),
+  ]),
+);
+
 const config: Config = {
   content: [
     "./app/**/*.{js,ts,jsx,tsx,mdx}",
@@ -24,6 +36,7 @@ const config: Config = {
           100: "#DDE0EA",
           50: "#E8E6E1",
         },
+        tint,
         accent: {
           cyan: "#00E5C7",
           amber: "#FFB547",