Change design of quote posts in web UI (#35584)

app/javascript/styles/mastodon/css_variables.scss

@@ -16,6 +16,7 @@
   --surface-background-color: #{darken($ui-base-color, 4%)};
   --surface-variant-background-color: #{$ui-base-color};
   --surface-variant-active-background-color: #{lighten($ui-base-color, 4%)};
+  --surface-border-color: #{lighten($ui-base-color, 8%)};
   --on-surface-color: #{color.adjust($ui-base-color, $alpha: -0.5)};
   --avatar-border-radius: 8px;
   --media-outline-color: #{rgba(#fcf8ff, 0.15)};