Harmonize persistent scrollbar styles (#31445)

app/javascript/styles/mastodon-light/diff.scss

@@ -1,10 +1,6 @@
 // Notes!
 // Sass color functions, "darken" and "lighten" are automatically replaced.
 
-html {
-  scrollbar-color: $ui-base-color rgba($ui-base-color, 0.25);
-}
-
 .simple_form .button.button-tertiary {
   color: $highlight-text-color;
 }
@@ -561,3 +557,14 @@ a.sparkline {
     color: $dark-text-color;
   }
 }
+
+@supports not selector(::-webkit-scrollbar) {
+  html {
+    scrollbar-color: rgba($action-button-color, 0.25)
+      var(--background-border-color);
+  }
+}
+
+::-webkit-scrollbar-thumb {
+  opacity: 0.25;
+}