Fix invisible borders on Rem and Ram themes

Both themes set borderColor == baseBackground in their IDE editor-scheme
override (a seamless look for code editors). For the web UI secondary
colour, fall back to secondaryBackground when borderColor would match
the page background exactly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/themeBuilder.ts

@@ -79,8 +79,14 @@ export function buildVariables(theme: ThemeConfig): Record<string, string> {
   const isDark = theme.colorScheme === 'dark';
 
   const primary = get(p, 'accentColor');
-  const secondary = get(p, 'borderColor', get(p, 'secondaryBackground'));
   const body = get(p, 'baseBackground');
+  // Some themes (e.g. Rem, Ram) set borderColor == baseBackground in their IDE
+  // editor-scheme override, creating an invisible-border effect suited for IDE
+  // editors but unusable in a web UI. Fall back to secondaryBackground instead.
+  const rawSecondary = get(p, 'borderColor', get(p, 'secondaryBackground'));
+  const secondary = rawSecondary === body
+    ? get(p, 'secondaryBackground', scaleLightness(body, isDark ? 0.08 : -0.08))
+    : rawSecondary;
   const fg = get(p, 'foregroundColor');
   const headerColor = get(p, 'headerColor');
   const contrastColor = get(p, 'contrastColor');