fix: use Headlamp theme system for drawer background

Replace hardcoded colors with --mui-palette-background-default which:
- Is fully opaque (fixes transparency issue)
- Automatically adapts to Headlamp's light/dark theme
- Matches pattern used throughout codebase
- Eliminates need for media query

Previous approach used hardcoded #ffffff/#1e1e1e which didn't
follow Headlamp's theme settings or custom themes.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

src/components/NamespacesListView.tsx

@@ -108,19 +108,13 @@ function NamespaceDetailPanel({ namespace, onClose }: NamespaceDetailPanelProps)
             top: 0;
             bottom: 0;
             width: 1000px;
-            background-color: #ffffff;
-            color: var(--mui-palette-text-primary, #000);
+            background-color: var(--mui-palette-background-default, #fafafa);
+            color: var(--mui-palette-text-primary);
             box-shadow: -2px 0 8px rgba(0,0,0,0.15);
             overflow-y: auto;
             z-index: 1200;
             padding: 20px;
           }
-          @media (prefers-color-scheme: dark) {
-            .${drawerClass} {
-              background-color: #1e1e1e;
-              color: var(--mui-palette-text-primary, #fff);
-            }
-          }
         `}
       </style>
       <div className={drawerClass}>