diff --git a/LESSONS.md b/LESSONS.md
index 8845ecb..bc115bf 100644
--- a/LESSONS.md
+++ b/LESSONS.md
@@ -1269,3 +1269,27 @@ every mode, and `outline` is already WCAG-lifted, so the high-contrast toggle
 reaches them too. Rule of thumb: if a control's fill is a near-surface tint, its
 legibility must come from a border or the saturated tone, never from the fill alone
 (this is the same "communicate via shape, not colour alone" rule the a11y notes state).
+
+---
+
+## 56. Never give an elevated Card a translucent container colour: the shadow ring shows through
+
+Done tasks and done/archived memos were tinted with
+`surfaceVariant.copy(alpha = 0.6f)` on a `Card` that still carried its 1dp light-mode
+elevation. Every active card looked fine; every muted card grew a thick grey border
+around a paler, smaller-radius inner rectangle, which nobody had drawn.
+
+The border is the platform shadow. HWUI renders an elevated node's shadow as a ring
+(penumbra out to the umbra edge) and leaves the umbra unfilled, on the assumption
+that the caster covers it. With an opaque fill that holds. With a 60% fill the ring
+shows through around the edge, while the unfilled umbra in the middle shows only the
+pale blend, so the card looks like a grey frame around a lighter inset panel. The
+inset roughly matched the content padding, which made it look like a deliberate
+(bad) inner border rather than a rendering artefact.
+
+Fix (`ui/theme/CardColors.kt`, `mutedCardContainer()`): keep the design intent but
+flatten the blend into an opaque colour with `compositeOver(colorScheme.background)`.
+Rule: any `Surface`/`Card` with `shadowElevation > 0` gets an opaque container. If a
+translucent look is genuinely wanted, drop the elevation to 0dp instead of relying on
+the fill to hide the shadow. Dark schemes here run at 0dp, which is why the bug only
+showed in light mode.
diff --git a/app/src/main/java/com/mapgie/dash/ui/components/ReminderCard.kt b/app/src/main/java/com/mapgie/dash/ui/components/ReminderCard.kt
index 6fb8e11..4154f1f 100644
--- a/app/src/main/java/com/mapgie/dash/ui/components/ReminderCard.kt
+++ b/app/src/main/java/com/mapgie/dash/ui/components/ReminderCard.kt
@@ -40,6 +40,7 @@ import com.mapgie.dash.ui.theme.StatusTone
 import com.mapgie.dash.ui.theme.badgeContainerColor
 import com.mapgie.dash.ui.theme.barColor
 import com.mapgie.dash.ui.theme.isDarkScheme
+import com.mapgie.dash.ui.theme.mutedCardContainer
 import com.mapgie.dash.ui.theme.spineColor
 import com.mapgie.dash.ui.theme.statusTone
 import com.mapgie.dash.ui.theme.textColor
@@ -95,10 +96,8 @@ fun ReminderCard(
             .fillMaxWidth()
             .padding(horizontal = inset),
         colors = CardDefaults.cardColors(
-            containerColor = if (muted)
-                MaterialTheme.colorScheme.surfaceVariant.copy(alpha = 0.6f)
-            else
-                MaterialTheme.colorScheme.surfaceVariant
+            containerColor = if (muted) mutedCardContainer()
+            else MaterialTheme.colorScheme.surfaceVariant
         ),
         elevation = CardDefaults.cardElevation(defaultElevation = if (dark) 0.dp else 1.dp)
     ) {
diff --git a/app/src/main/java/com/mapgie/dash/ui/components/TaskCard.kt b/app/src/main/java/com/mapgie/dash/ui/components/TaskCard.kt
index eaf3630..4eebe0e 100644
--- a/app/src/main/java/com/mapgie/dash/ui/components/TaskCard.kt
+++ b/app/src/main/java/com/mapgie/dash/ui/components/TaskCard.kt
@@ -44,6 +44,7 @@ import com.mapgie.dash.ui.theme.StatusTone
 import com.mapgie.dash.ui.theme.badgeContainerColor
 import com.mapgie.dash.ui.theme.barColor
 import com.mapgie.dash.ui.theme.isDarkScheme
+import com.mapgie.dash.ui.theme.mutedCardContainer
 import com.mapgie.dash.ui.theme.statusTone
 import com.mapgie.dash.ui.theme.textColor
 import java.time.Instant
@@ -97,7 +98,7 @@ fun TaskCard(
         colors = CardDefaults.cardColors(
             containerColor = when {
                 zenMode -> MaterialTheme.colorScheme.surfaceContainerLow
-                isDone -> MaterialTheme.colorScheme.surfaceVariant.copy(alpha = 0.6f)
+                isDone -> mutedCardContainer()
                 else -> MaterialTheme.colorScheme.surfaceVariant
             }
         ),
diff --git a/app/src/main/java/com/mapgie/dash/ui/theme/CardColors.kt b/app/src/main/java/com/mapgie/dash/ui/theme/CardColors.kt
new file mode 100644
index 0000000..2fbe50d
--- /dev/null
+++ b/app/src/main/java/com/mapgie/dash/ui/theme/CardColors.kt
@@ -0,0 +1,23 @@
+package com.mapgie.dash.ui.theme
+
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.graphics.compositeOver
+
+/**
+ * Fill for a muted list card (a done task, a done or archived memo): 60% of
+ * `surfaceVariant` flattened onto the page background so the card reads as faded
+ * next to its active siblings.
+ *
+ * The blend is composited into an opaque colour on purpose. A translucent
+ * `containerColor` on an elevated `Card` lets the platform shadow show through
+ * the fill, and because the shadow is drawn as a ring with an unfilled umbra
+ * the card picks up a thick grey border around a paler inner rectangle. See
+ * `LESSONS.md` #56.
+ */
+@Composable
+fun mutedCardContainer(): Color =
+    MaterialTheme.colorScheme.surfaceVariant
+        .copy(alpha = 0.6f)
+        .compositeOver(MaterialTheme.colorScheme.background)
diff --git a/changelog/unreleased/muted-card-border.json b/changelog/unreleased/muted-card-border.json
new file mode 100644
index 0000000..8c2eff6
--- /dev/null
+++ b/changelog/unreleased/muted-card-border.json
@@ -0,0 +1,4 @@
+{
+  "bump": "patch",
+  "fixed": ["Done tasks and done or archived memos no longer show a thick grey border around a paler inner rectangle; the faded card fill is now a solid colour."]
+}