[Glitch] Minor Wrapstodon tweaks, add stub Storybook page

Port 91500a7f53 to glitch-soc

Signed-off-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/flavours/glitch/styles/mastodon/theme/index.scss

@@ -6,7 +6,7 @@
 html {
   @include base.palette;
 
-  &[data-user-theme='system'] {
+  &:where([data-user-theme='system']) {
     color-scheme: dark light;
 
     @media (prefers-color-scheme: dark) {