feat(theme): add Theme (#741)

* feat(theme): add Theme

* fix(theme): fix broken type test

* docs(theme): add examples

* docs(theme): add description, update carbon theme link [ci skip]

* docs: pre-wrap type code snippet [ci skip]

docs/src/pages/components/Theme.svx

@@ -0,0 +1,46 @@
+<script>
+  import { Theme } from "carbon-components-svelte";
+  import Preview from "../../components/Preview.svelte";
+</script>
+
+This utility component dynamically updates the Carbon theme on the client-side using CSS variables.
+
+### Default
+
+<FileSource src="/framed/Theme/Theme" />
+
+### Persist locally
+
+Set `persist` to `true` to persist the theme locally using the [Window.localStorage API](https://developer.mozilla.org/en-US/docs/Web/API/Window/localStorage).
+
+<FileSource src="/framed/Theme/ThemePersist" />
+
+### Custom theme
+
+Define keys and values in the `tokens` prop that override default Carbon theme tokens. Refer to the [Carbon website](https://carbondesignsystem.com/guidelines/themes/overview#customizing-a-theme) for guidance on customizing a theme using token values.
+
+<FileSource src="/framed/Theme/ThemeTokens" />
+
+### Theme toggle
+
+Set `render` to `"toggle"` to render a toggle switch to control the theme.
+
+<FileSource src="/framed/Theme/ThemeToggle" />
+
+### Theme toggle (custom)
+
+Customize the toggle using the `toggle` prop.
+
+<FileSource src="/framed/Theme/ThemeToggleCustom" />
+
+### Theme select
+
+Set `render` to `"select"` to render a select dropdown to control the theme.
+
+<FileSource src="/framed/Theme/ThemeSelect" />
+
+### Theme select (custom)
+
+Customize the select using the `select` prop.
+
+<FileSource src="/framed/Theme/ThemeSelectCustom" />