Reserve the red announcement banner for prerelease and fix its contrast (#2276)

Author: cwickham

_quarto-prerelease-docs.yml

@@ -18,4 +18,5 @@ format:
   html:
     toc: true
     theme:
-      light: [cosmo, theme.scss, theme-prerelease-docs.scss]
+      light: [cosmo, theme.scss, theme-prerelease-banner.scss]
+      dark: [cosmo, theme-dark.scss, theme-prerelease-banner.scss]

theme-dark.scss

@@ -183,7 +183,7 @@ details > summary {
   font-size: 1em;
   font-weight: bold;
   color: $white;
-  background-color: #ff0039; 
+  background-color: #447099; 
 }
 
 #quarto-announcement a {

theme-prerelease-banner.scss

@@ -0,0 +1,11 @@
+/*-- scss:rules --*/
+
+// Red marks the prerelease site, in light and dark mode.
+// #e60033 gives white text 4.76:1 (WCAG 1.4.3 needs 4.5:1).
+// Cosmo's $danger (#ff0039) gives only 3.95:1.
+#quarto-announcement {
+  font-size: 1em;
+  font-weight: bold;
+  color: $white;
+  background-color: #e60033;
+}