chore(examples): extract Theme component and persist to localStorage

examples/webpack/src/components/Theme.svelte

@@ -1,7 +1,55 @@
 <script>
+  export let persist = false;
+  export let persistKey = "theme";
   export let theme = "white";
+  export const themes = ["white", "g10", "g90", "g100"];
 
-  $: document.documentElement.setAttribute("theme", theme);
+  import { onMount, afterUpdate, setContext } from "svelte";
+  import { writable, derived } from "svelte/store";
+
+  const isValidTheme = (value) => themes.includes(value);
+  const isDark = (value) =>
+    isValidTheme(value) && (value === "g90" || value === "g100");
+
+  const dark = writable(isDark(theme));
+  const light = derived(dark, (_) => !_);
+
+  setContext("Theme", {
+    updateVar: (name, value) => {
+      document.documentElement.style.setProperty(name, value);
+    },
+    dark,
+    light,
+  });
+
+  onMount(() => {
+    try {
+      const persisted_theme = localStorage.getItem(persistKey);
+
+      if (isValidTheme(persisted_theme)) {
+        theme = persisted_theme;
+      }
+    } catch (error) {
+      console.error(error);
+    }
+  });
+
+  afterUpdate(() => {
+    if (isValidTheme(theme)) {
+      document.documentElement.setAttribute("theme", theme);
+      if (persist) {
+        localStorage.setItem(persistKey, theme);
+      }
+    } else {
+      console.warn(
+        `"${theme}" is not a valid Carbon theme. Choose from available themes: ${JSON.stringify(
+          themes
+        )}`
+      );
+    }
+  });
+
+  $: dark.set(isDark(theme));
 </script>
 
 <slot />