Replace radio/checkbox PNG assets with pure CSS (Mint-Y)

[sporteka2]

Target: Mint-Y GTK3 radio buttons and checkboxes
Current: 16×16 PNG + 32×32 @2x PNG for each state (×5 states × 2 widget types ≈ 20+ PNG files)
Proposal: Replace PNG assets with pure CSS via appearance: none + border-radius + radial-gradient

Current assets

Each radio/checkbox has up to 5 state variants, each with 1× and 2× PNGs:

assets/radio-unchecked.png          (16×16)
assets/radio-unchecked@2.png        (32×32)
assets/radio-unchecked-dark.png
assets/radio-checked.png
assets/radio-checked-dark.png
assets/radio-unchecked-selected.png
assets/radio-checked-selected.png
assets/radio-indeterminate.png
... (same for checkboxes)

Proposed CSS approach (conceptual)

radio,
check {
  appearance: none;
  -gtk-icon-source: none;
  min-width: 16px;
  min-height: 16px;
  border-radius: 50%;
  border: 2px solid @borders;
  background: @theme_base_color;
  transition: border-color 200ms, background 200ms;
}

radio:checked {
  border-color: @theme_selected_bg_color;
  background: radial-gradient(
    circle at 50% 50%,
    @theme_base_color 1.5px,
    @theme_selected_bg_color 2.5px
  );
}

Benefits

• ~20 fewer PNG files — smaller package size
• HiDPI native — no separate @2x assets, renders crisp at any scale
• Easier theming — colors come from CSS variables, no need to regenerate PNGs for each accent
• Animations — smooth transition between states
• Hover state — trivially added via CSS if desired
• Focus ring — :focus-visible outline can be styled directly