Improve card header visibility in brutalism template dark mode

- Keep card-header background yellow in both light and dark modes using
  --accent-tertiary variable (#ffff00 light, #ffff33 dark)
- Set card-title text to black in dark mode for optimal contrast against
  yellow background
- Remove dark mode overrides that changed header background to gray

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

templates/brutalism.html

@@ -154,17 +154,6 @@
             background: var(--accent-tertiary);
         }
 
-        [data-theme="dark"] .card-header,
-        .dark-theme .card-header {
-            background: var(--bg-tertiary);
-        }
-
-        @media (prefers-color-scheme: dark) {
-            :root:not([data-theme="light"]) .card-header {
-                background: var(--bg-tertiary);
-            }
-        }
-
         .card-title {
             font-size: 18px;
             font-weight: 900;
@@ -173,6 +162,16 @@
             color: var(--text-primary);
         }
 
+        [data-theme="dark"] .card-title {
+            color: #000000;
+        }
+
+        @media (prefers-color-scheme: dark) {
+            :root:not([data-theme="light"]) .card-title {
+                color: #000000;
+            }
+        }
+
         .card-content {
             padding: 20px;
         }