Make header separator lighter in dark mode

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: kylemclaren

src/styles/custom.css

@@ -505,6 +505,11 @@ header.header {
   border-bottom-color: var(--sl-color-hairline);
 }
 
+/* Lighter separator between social icons and theme select in dark mode */
+[data-theme='dark'] header.header .social-icons::after {
+  border-color: rgba(255, 255, 255, 0.2);
+}
+
 /* Remove borders between sections (except header) */
 .sidebar-pane {
   border-inline-end: none;