docs(theme): add "Exported themes constant" example

Author: metonym

docs/src/pages/components/Theme.svx

@@ -63,4 +63,28 @@ Set `render` to `"select"` to display a dropdown menu for selecting from all ava
 
 Customize the select dropdown using the `select` prop. You can specify which themes to include, customize labels, and adjust other select properties.
 
-<FileSource src="/framed/Theme/ThemeSelectCustom" />
+<FileSource src="/framed/Theme/ThemeSelectCustom" />
+
+## Exported themes constant
+
+The `themes` constant is exported from the Theme component, providing programmatic access to all available theme keys and their human-readable labels. This is useful when building custom theme UIs or dynamically generating theme options.
+
+```js
+import { themes } from "carbon-components-svelte";
+
+// themes = {
+//   white: "White",
+//   g10: "Gray 10",
+//   g80: "Gray 80",
+//   g90: "Gray 90",
+//   g100: "Gray 100"
+// }
+
+// Get all theme keys programmatically
+const themeKeys = Object.keys(themes); // ["white", "g10", "g80", "g90", "g100"]
+
+// Build custom theme selectors with correct labels
+Object.entries(themes).forEach(([key, label]) => {
+  console.log(`${key}: ${label}`); // "white: White", etc.
+});
+```