feat: custom scrollbar styling — thin, dark, brand cyan hover

6px wide, rgba(48,54,61) thumb (--border at 60%), cyan hover/active.
Applied globally via * selector — covers sidebar, grid, settings, and
any future scrollable area. Firefox (scrollbar-width/color) + WebKit
(::-webkit-scrollbar) cross-browser.

muxplex/frontend/style.css

@@ -60,6 +60,43 @@
   box-sizing: border-box;
 }
 
+/* ============================================================
+   Custom scrollbar — thin, dark, matches theme
+   ============================================================ */
+
+/* Firefox */
+* {
+  scrollbar-width: thin;
+  scrollbar-color: rgba(48, 54, 61, 0.6) transparent;
+}
+
+/* WebKit (Chrome, Edge, Safari) */
+*::-webkit-scrollbar {
+  width: 6px;
+  height: 6px;
+}
+
+*::-webkit-scrollbar-track {
+  background: transparent;
+}
+
+*::-webkit-scrollbar-thumb {
+  background: rgba(48, 54, 61, 0.6);
+  border-radius: 3px;
+}
+
+*::-webkit-scrollbar-thumb:hover {
+  background: rgba(0, 200, 200, 0.35);  /* --accent at 35% */
+}
+
+*::-webkit-scrollbar-thumb:active {
+  background: rgba(0, 200, 200, 0.55);  /* --accent at 55% */
+}
+
+*::-webkit-scrollbar-corner {
+  background: transparent;
+}
+
 /* Base */
 html,
 body {

muxplex/tests/test_frontend_css.py

@@ -2077,3 +2077,16 @@ def test_css_multi_device_fields_transition() -> None:
         "#multi-device-fields must have a transition property for smooth enable/disable animation"
     )
     assert "opacity" in body, "#multi-device-fields transition must include opacity"
+
+
+# ============================================================
+# Custom scrollbar styling (thin, dark, brand cyan hover)
+# ============================================================
+
+
+def test_custom_scrollbar_styles():
+    """Custom scrollbar styling must exist for both WebKit and Firefox."""
+    css = read_css()
+    assert "scrollbar-width: thin" in css, "Firefox scrollbar-width must be set"
+    assert "::-webkit-scrollbar" in css, "WebKit scrollbar rules must exist"
+    assert "border-radius: 3px" in css or "border-radius:3px" in css, "scrollbar thumb must be rounded"