Give secondary buttons their own text colour

.pixel-btn inherited its text colour. Inside the update and install
pills, which set near-black on-fill text for their coloured background,
the "later" button put that near-black on a card that is dark purple on
Halloween: 1.2:1, effectively invisible. Now 13:1. Pink hid it because
its card is white.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: Kjubikstronk

src/theme.css

@@ -163,6 +163,10 @@
     font-weight: 700;
     border: 3px solid var(--color-line);
     background: var(--color-card);
+    /* Set, not inherited. A button inside a coloured banner (the update and
+       install pills) otherwise took the banner's on-fill text — near-black —
+       onto a card that is dark purple on Halloween, and read at 1.2:1. */
+    color: var(--color-text);
     box-shadow: 4px 4px 0 var(--color-shadow);
     transition:
       transform 90ms var(--ease-snap),