Theme the sidebar scrollbar

apps/web/src/index.css

@@ -158,6 +158,32 @@
     border-radius: 3px;
   }
 
+  /* The expanded sidebar owns its vertical scroll rather than the page. Keep
+     that scrollbar inside the sidebar palette: the browser's native light
+     track otherwise becomes a bright gutter in dark mode. */
+  [data-sidebar='content'] {
+    scrollbar-color: hsl(var(--sidebar-border)) transparent;
+    scrollbar-width: thin;
+  }
+
+  [data-sidebar='content']::-webkit-scrollbar {
+    width: 6px;
+  }
+
+  [data-sidebar='content']::-webkit-scrollbar-track,
+  [data-sidebar='content']::-webkit-scrollbar-corner {
+    background: transparent;
+  }
+
+  [data-sidebar='content']::-webkit-scrollbar-thumb {
+    background-color: hsl(var(--sidebar-border));
+    border-radius: 9999px;
+  }
+
+  [data-sidebar='content']::-webkit-scrollbar-thumb:hover {
+    background-color: hsl(var(--muted) / 0.55);
+  }
+
   /* Touch targets. 44px is the documented iOS minimum and the difference
      between a usable and a frustrating phone experience. */
   .tap {