fix: match main's announcement banner colours

The main site's banner is now the brand blue in dark mode too, as on
main. The prerelease banner moves to prerelease-banner.scss with
#e60033, which gives white text 4.76:1, and the prerelease profile
applies it to the dark theme as well.

Author: mcanouil

_extensions/quartoorg/scss/partials/_dark-rules.scss

@@ -95,7 +95,7 @@ details > summary {
   font-size: 1em;
   font-weight: bold;
   color: $white;
-  background-color: #ff0039; 
+  background-color: #447099; 
 }
 
 #quarto-announcement a {

theme-prerelease-banner.scss …ns/quartoorg/scss/prerelease-banner.scsstheme-prerelease-banner.scss renamed to _extensions/quartoorg/scss/prerelease-banner.scss theme-prerelease-banner.scss renamed to _extensions/quartoorg/scss/prerelease-banner.scss

@@ -17,4 +17,5 @@ website:
 format:
   quartoorg-html:
     theme:
-      light: [_extensions/quartoorg/scss/prerelease.scss]
+      light: [_extensions/quartoorg/scss/prerelease-banner.scss]
+      dark: [_extensions/quartoorg/scss/prerelease-banner.scss]