Theme

npm install next-themes

src/components/theme-provider.tsx

"use client";

import * as React from "react";
import { ThemeProvider as NextThemesProvider } from "next-themes";

export function ThemeProvider({
  children,
  ...props
}: React.ComponentProps<typeof NextThemesProvider>) {
  const [mounted, setMounted] = React.useState(false);
  React.useEffect(() => setMounted(true), []);

  if (!mounted) {
    return <div style={{ visibility: "hidden" }}>{children}</div>;
  }
  return <NextThemesProvider {...props}>{children}</NextThemesProvider>;
}

src/app/layout.tsx

import { ThemeProvider } from "@/components/theme-provider";

export default function RootLayout({ children }: RootLayoutProps) {
  return (
    <>
      <html lang="en" suppressHydrationWarning>
        <head />
        <body>
          <ThemeProvider
            attribute="class"
            defaultTheme="system"
            enableSystem
            disableTransitionOnChange
          >
            {children}
          </ThemeProvider>
        </body>
      </html>
    </>
  );
}

npx shadcn@latest add dropdown-menu

src/components/mode-toggle.tsx

"use client";

import * as React from "react";
import { Moon, Sun } from "lucide-react";
import { useTheme } from "next-themes";

import { Button } from "@/components/ui/button";
import {
  DropdownMenu,
  DropdownMenuContent,
  DropdownMenuItem,
  DropdownMenuTrigger,
} from "@/components/ui/dropdown-menu";

export function ModeToggle() {
  const { setTheme } = useTheme();

  return (
    <DropdownMenu>
      <DropdownMenuTrigger asChild>
        <Button variant="outline" size="icon">
          <Sun className="h-[1.2rem] w-[1.2rem] scale-100 rotate-0 transition-all dark:scale-0 dark:-rotate-90" />
          <Moon className="absolute h-[1.2rem] w-[1.2rem] scale-0 rotate-90 transition-all dark:scale-100 dark:rotate-0" />
          <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>
  );
}