fix(ui): address custom theme review cleanup

ui/src/styles/config.css

@@ -559,26 +559,12 @@
   transform: translateY(-1px);
 }
 
-.settings-theme-card:disabled {
-  cursor: not-allowed;
-}
-
 .settings-theme-card--active {
   border-color: color-mix(in srgb, var(--accent) 35%, transparent);
   background: color-mix(in srgb, var(--accent) 10%, var(--bg-elevated));
   box-shadow: 0 0 0 1px color-mix(in srgb, var(--accent) 14%, transparent);
 }
 
-.settings-theme-card--disabled {
-  opacity: 0.6;
-}
-
-.settings-theme-card--disabled:hover {
-  border-color: var(--border);
-  background: var(--bg);
-  transform: none;
-}
-
 .settings-theme-card__icon,
 .settings-theme-card__check {
   display: inline-flex;

ui/src/ui/custom-theme.test.ts

@@ -7,6 +7,7 @@ import {
   parseImportedCustomTheme,
   syncCustomThemeStyleTag,
 } from "./custom-theme.ts";
+import type { ImportedCustomTheme } from "./custom-theme.ts";
 
 function createTweakcnPayload() {
   return {
@@ -183,6 +184,14 @@ describe("custom theme import helpers", () => {
     expect(css).toContain("--bg: oklch(0.98 0.01 120);");
   });
 
+  it("throws when stored custom theme tokens are missing", () => {
+    const theme = { ...createImportedTheme(), light: undefined } as unknown as ImportedCustomTheme;
+
+    expect(() => buildCustomThemeStyles(theme)).toThrow(
+      "Stored custom theme is missing required tokens.",
+    );
+  });
+
   it("parses stored imported themes and rejects malformed records", () => {
     const imported = createImportedTheme();
 

ui/src/ui/custom-theme.ts

@@ -504,7 +504,7 @@ export function buildCustomThemeStyles(theme: ImportedCustomTheme) {
   const light = normalizeStoredTokenMap(theme.light);
   const dark = normalizeStoredTokenMap(theme.dark);
   if (!light || !dark) {
-    return "";
+    throw new Error("Stored custom theme is missing required tokens.");
   }
   const renderDeclarations = (modeTokens: ThemeTokenMap) =>
     MODE_TOKEN_ORDER.map((key) => `  --${key}: ${modeTokens[key]};`).join("\n");