Add theme toggle functionality with ThemeProvider and ModeToggle component

src/components/mode-toggle.tsx

@@ -0,0 +1,37 @@
+import { Moon, Sun } from "lucide-react";
+
+import { Button } from "@/components/ui/button";
+import {
+  DropdownMenu,
+  DropdownMenuContent,
+  DropdownMenuItem,
+  DropdownMenuTrigger,
+} from "@/components/ui/dropdown-menu";
+import { useTheme } from "@/components/theme-provider";
+
+export function ModeToggle() {
+  const { setTheme } = useTheme();
+
+  return (
+    <DropdownMenu>
+      <DropdownMenuTrigger asChild>
+        <Button variant="outline" size="icon">
+          <Sun className="h-[1.2rem] w-[1.2rem] rotate-0 scale-100 transition-all dark:-rotate-90 dark:scale-0" />
+          <Moon className="absolute h-[1.2rem] w-[1.2rem] rotate-90 scale-0 transition-all dark:rotate-0 dark:scale-100" />
+          <span className="sr-only">Toggle theme</span>
+        </Button>
+      </DropdownMenuTrigger>
+      <DropdownMenuContent align="end">
+        <DropdownMenuItem onClick={() => setTheme("light")}>
+          Light
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme("dark")}>
+          Dark
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme("system")}>
+          System
+        </DropdownMenuItem>
+      </DropdownMenuContent>
+    </DropdownMenu>
+  );
+}

src/components/theme-provider.tsx

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+import { createContext } from "preact";
+import { useContext, useEffect, useState } from "preact/hooks";
+
+type Theme = "dark" | "light" | "system";
+
+type ThemeProviderProps = {
+  children: preact.VNode;
+  defaultTheme?: Theme;
+  storageKey?: string;
+};
+
+type ThemeProviderState = {
+  theme: Theme;
+  setTheme: (theme: Theme) => void;
+};
+
+const initialState: ThemeProviderState = {
+  theme: "system",
+  setTheme: () => null,
+};
+
+const ThemeProviderContext = createContext<ThemeProviderState>(initialState);
+
+export function ThemeProvider({
+  children,
+  defaultTheme = "system",
+  storageKey = "vite-ui-theme",
+  ...props
+}: ThemeProviderProps) {
+  const [theme, setTheme] = useState<Theme>(
+    () => (localStorage.getItem(storageKey) as Theme) || defaultTheme
+  );
+
+  useEffect(() => {
+    const root = window.document.documentElement;
+
+    root.classList.remove("light", "dark");
+
+    if (theme === "system") {
+      const systemTheme = window.matchMedia("(prefers-color-scheme: dark)")
+        .matches
+        ? "dark"
+        : "light";
+
+      root.classList.add(systemTheme);
+      return;
+    }
+
+    root.classList.add(theme);
+  }, [theme]);
+
+  const value = {
+    theme,
+    setTheme: (theme: Theme) => {
+      localStorage.setItem(storageKey, theme);
+      setTheme(theme);
+    },
+  };
+
+  return (
+    <ThemeProviderContext.Provider {...props} value={value}>
+      {children}
+    </ThemeProviderContext.Provider>
+  );
+}
+
+export const useTheme = () => {
+  const context = useContext(ThemeProviderContext);
+
+  if (context === undefined)
+    throw new Error("useTheme must be used within a ThemeProvider");
+
+  return context;
+};