Consistent scrollbar styling across browsers

- colors.css: added --scrollbar-thumb, --scrollbar-thumb-hover, --scrollbar-track, --scrollbar-corner custom properties
- base.css: consolidated all scrollbar rules (WebKit ::-webkit-scrollbar* + Firefox scrollbar-width/color), added ::-webkit-scrollbar-corner, changed selector from * to html (scrollbar-width inherits), added scrollbar-gutter: stable
- admin.css: removed redundant scrollbar-width: thin on header nav
- toc.css: removed redundant scrollbar-width: thin on desktop .toc

app/public/assets/css/admin.css

@@ -14,7 +14,6 @@
 .admin-body header nav {
   overflow-x: auto;
   white-space: nowrap;
-  scrollbar-width: thin;
 }
 .admin-body header nav .nav-right-links {
   flex-shrink: 0;

app/public/assets/css/base.css

@@ -11,30 +11,22 @@ body {
   overflow: hidden;
 }
 
-/* ── Discreet scrollbars ─────────────────────────────────────────── */
-/* WebKit browsers */
+/* ── Discreet scrollbars (WebKit / Blink / Firefox) ──────────────── */
+/* CSS custom properties defined in colors.css:
+   --scrollbar-thumb, --scrollbar-thumb-hover, --scrollbar-track, --scrollbar-corner */
 ::-webkit-scrollbar {
-  width: 5px;
-  height: 5px;
+  width:  var(--scrollbar-width, 5px);
+  height: var(--scrollbar-width, 5px);
 }
+::-webkit-scrollbar-track        { background: var(--scrollbar-track); }
+::-webkit-scrollbar-thumb        { background: var(--scrollbar-thumb); border-radius: 3px; }
+::-webkit-scrollbar-thumb:hover  { background: var(--scrollbar-thumb-hover); }
+::-webkit-scrollbar-corner       { background: var(--scrollbar-corner); }
 
-::-webkit-scrollbar-track {
-  background: transparent;
-}
-
-::-webkit-scrollbar-thumb {
-  background: var(--text-tertiary);
-  border-radius: 3px;
-}
-
-::-webkit-scrollbar-thumb:hover {
-  background: var(--text-secondary);
-}
-
-/* Firefox */
-* {
+html {
   scrollbar-width: thin;
-  scrollbar-color: var(--text-tertiary) transparent;
+  scrollbar-color: var(--scrollbar-thumb) var(--scrollbar-track);
+  scrollbar-gutter: stable;
 }
 
 body {

app/public/assets/css/colors.css

@@ -15,6 +15,13 @@
 	--text-secondary: #666666;
 	--text-tertiary: #999999;
 
+	/* Scrollbar */
+	--scrollbar-width: 5px;
+	--scrollbar-thumb: var(--text-tertiary);
+	--scrollbar-thumb-hover: var(--text-secondary);
+	--scrollbar-track: transparent;
+	--scrollbar-corner: transparent;
+
 	/* Borders */
 	--border-primary: #dddddd;
 	--border-secondary: #cccccc;

app/public/assets/css/components/toc.css

@@ -119,7 +119,6 @@
 		max-height: calc(100vh - var(--space-xl) - var(--space-2xl));
 		overflow-y: auto;
 		overscroll-behavior: contain;
-		scrollbar-width: thin;
 	}
 
 	.toc summary {