panels are a themed surface, not three literal rgba pairs

The cornerstone of the layout was the least themeable thing in it. Measured before
touching anything, which is the only reason this is the right fix:

  duck-dark  radius=8px  bg=rgba(255,255,255,0.12)  border=rgba(0,0,0,0.25)  blur=24px
  plain      radius=0px  bg=rgba(255,255,255,0.12)  border=rgba(0,0,0,0.25)  blur=24px

So the RADIUS was already following `--radius` and I had been telling the owner
otherwise — the rounding in the Plain screenshot was something else. What did not
follow anything was the surface: three literal rgba pairs and a fixed `backdrop-blur-xl`
in the component.

Now:

  plain      radius=0px  bg=rgb(255,255,255)  border=rgb(0,0,0)  blur=0px

Two distinctions the old code drew in literals are kept as tokens rather than
flattened, because both are real and a theme should be able to revisit them.
`--panel-raised-*` is the fullscreen surface: DENSER glass, not darker, because panel
chrome is dark text and a heavy dark fill hid every icon. `--panel-border-bare` is for a
panel with no header, which has nothing else separating it from what it sits on.

`--panel-blur` earns its place beyond theming: a flat theme was paying for a 24px
backdrop blur it did not want, and on a solid panel the compositor does that work for
nothing.

── A near miss worth recording ──

The first attempt silently changed nothing. The patch matched on four spaces of
indentation where the file has two, `str.replace` reported no error, the build
succeeded, and `panel-blur` DID apply — so the class landed while the surface did not.
It looked like a partial success and would have shipped as one. Caught by measuring
computed styles again rather than trusting the edit; the fix now asserts the
replacement changed the file.

950 pass / 7 fail unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/workspaces/theme/css/chrome.css

@@ -83,3 +83,9 @@
 .header-ink-border {
   border-color: color-mix(in srgb, var(--header-fg) 40%, transparent);
 }
+
+/* Panel glass. `backdrop-blur-xl` was a fixed 24px, so a flat theme still paid for a blur it did not want —
+   and on a solid panel the browser composites it for nothing. */
+.panel-blur {
+  backdrop-filter: blur(var(--panel-blur));
+}

src/workspaces/theme/css/themes/duck-dark.css

@@ -67,8 +67,12 @@
   --glass-bg: rgba(0, 0, 0, 0.2);
   --glass-border: rgba(255, 255, 255, 0.1);
 
-  --panel-bg: rgba(0, 0, 0, 0.2);
-  --panel-border: rgba(255, 255, 255, 0.1);
+  --panel-bg: rgba(255, 255, 255, 0.12);
+  --panel-border: rgba(255, 255, 255, 0.2);
+  --panel-border-bare: rgba(0, 0, 0, 0.25);
+  --panel-raised-bg: rgba(255, 255, 255, 0.25);
+  --panel-raised-border: rgba(255, 255, 255, 0.35);
+  --panel-blur: 24px;
   --panel-radius: 0.75rem;
   --panel-header-bg: rgba(0, 0, 0, 0.15);
   --panel-header-fg: hsl(var(--foreground));

src/workspaces/theme/css/themes/duck-light.css

@@ -72,8 +72,14 @@
   --glass-bg: rgba(255, 255, 255, 0.1);
   --glass-border: rgba(255, 255, 255, 0.2);
 
-  --panel-bg: rgba(255, 255, 255, 0.1);
+  --panel-bg: rgba(255, 255, 255, 0.12);
   --panel-border: rgba(255, 255, 255, 0.2);
+  /* A panel with no header has nothing to separate it from what it sits on, so it takes a harder edge. */
+  --panel-border-bare: rgba(0, 0, 0, 0.25);
+  /* Fullscreen is DENSER glass, not darker: panel chrome is dark text, and a heavy dark fill hid it. */
+  --panel-raised-bg: rgba(255, 255, 255, 0.25);
+  --panel-raised-border: rgba(255, 255, 255, 0.35);
+  --panel-blur: 24px;
   --panel-radius: 0.75rem;
   --panel-header-bg: rgba(0, 0, 0, 0.05);
   --panel-header-fg: hsl(var(--foreground));

src/workspaces/theme/css/themes/plain.css

@@ -72,6 +72,10 @@
 
   --panel-bg: #ffffff;
   --panel-border: #000000;
+  --panel-border-bare: #000000;
+  --panel-raised-bg: #ffffff;
+  --panel-raised-border: #000000;
+  --panel-blur: 0px;
   --panel-radius: 0;
   --panel-header-bg: #f2f2f2;
   --panel-header-fg: #000000;