WebUI: Use thin scrollbars

Thin scrollbars are now used if they are supported by user's browser. The main goal was to make them less intrusive in Chrome-likes on some platforms.

PR #21763.
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Co-authored-by: Chocobo1 <Chocobo1@users.noreply.github.com>

src/webui/www/private/css/style.css

@@ -75,6 +75,18 @@ ol {
     list-style: none;
 }
 
+/* Scrollbars */
+
+@supports (scrollbar-width: auto) and (selector(::-webkit-scrollbar)) {
+    :root[slick-uniqueid], /* targets iframe mocha dialogs */
+    .dynamicTableDiv,
+    .mochaContentWrapper,
+    .panel,
+    #rssDetailsView {
+        scrollbar-width: thin;
+    }
+}
+
 /* Forms */
 
 input[type="text"],