Fix admin sidebar contrast: muted text now visually distinct from regular text

src/lib/theme-css.ts

@@ -59,7 +59,7 @@ export function themePaletteCss(selector: string, palette: ThemePalette): string
     `--color-text-disabled-readable:${semantic.byName.disabled};`,
     `--admin-sidebar-background:${palette.color_navbar};`,
     `--admin-sidebar-text-readable:${semantic.byName.sidebar};`,
-    `--admin-sidebar-muted-readable:${semantic.byName.sidebar};`,
+    `--admin-sidebar-muted-readable:color-mix(in srgb, ${semantic.byName.sidebar}, ${palette.color_navbar} 38%);`,
     `--admin-input-text-readable:${semantic.byName.input};`,
     `--admin-canvas:${admin.canvas};`,
     `--admin-surface:${admin.surface};`,