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Adding dark mode to your TanStack Start app.

Create a theme provider

TanStack Start uses ScriptOnce from @tanstack/react-router to inject a script that runs before React hydrates, preventing flash of unstyled content (FOUC).

components/theme-provider.tsx
import { createContext, useContext, useEffect, useState } from "react"
import { ScriptOnce } from "@tanstack/react-router"
 
type Theme = "dark" | "light" | "system"
 
type ThemeProviderProps = {
  children: React.ReactNode
  defaultTheme?: Theme
  storageKey?: string
}
 
type ThemeProviderState = {
  theme: Theme
  setTheme: (theme: Theme) => void
}
 
function getThemeScript(storageKey: string, defaultTheme: Theme) {
  const key = JSON.stringify(storageKey)
  const fallback = JSON.stringify(defaultTheme)
 
  return `(function(){try{var t=localStorage.getItem(${key});if(t!=='light'&&t!=='dark'&&t!=='system'){t=${fallback}}var d=matchMedia('(prefers-color-scheme: dark)').matches;var r=t==='system'?(d?'dark':'light'):t;var e=document.documentElement;e.classList.add(r);e.style.colorScheme=r}catch(e){}})();`
}
 
const ThemeProviderContext = createContext<ThemeProviderState>({
  theme: "system",
  setTheme: () => {},
})
 
function applyTheme(theme: Theme) {
  const root = document.documentElement
  root.classList.remove("light", "dark")
 
  const resolved =
    theme === "system"
      ? window.matchMedia("(prefers-color-scheme: dark)").matches
        ? "dark"
        : "light"
      : theme
 
  root.classList.add(resolved)
  root.style.colorScheme = resolved
}
 
export function ThemeProvider({
  children,
  defaultTheme = "system",
  storageKey = "theme",
}: ThemeProviderProps) {
  const [theme, setThemeState] = useState<Theme>(defaultTheme)
  const [mounted, setMounted] = useState(false)
 
  useEffect(() => {
    const stored = localStorage.getItem(storageKey)
    setThemeState(
      stored === "light" || stored === "dark" || stored === "system"
        ? stored
        : defaultTheme
    )
    setMounted(true)
  }, [defaultTheme, storageKey])
 
  useEffect(() => {
    if (!mounted) return
    applyTheme(theme)
  }, [theme, mounted])
 
  useEffect(() => {
    if (!mounted || theme !== "system") return
 
    const media = window.matchMedia("(prefers-color-scheme: dark)")
    const onChange = () => applyTheme("system")
    media.addEventListener("change", onChange)
    return () => media.removeEventListener("change", onChange)
  }, [theme, mounted])
 
  const setTheme = (next: Theme) => {
    localStorage.setItem(storageKey, next)
    setThemeState(next)
  }
 
  return (
    <ThemeProviderContext value={{ theme, setTheme }}>
      <ScriptOnce>{getThemeScript(storageKey, defaultTheme)}</ScriptOnce>
      {children}
    </ThemeProviderContext>
  )
}
 
export function useTheme() {
  const context = useContext(ThemeProviderContext)
  if (context === undefined)
    throw new Error("useTheme must be used within a ThemeProvider")
  return context
}

Wrap your root layout

Add the ThemeProvider to your root layout and add the suppressHydrationWarning prop to the html tag.

src/routes/__root.tsx
import {
  createRootRoute,
  HeadContent,
  Outlet,
  Scripts,
} from "@tanstack/react-router"
 
import { ThemeProvider } from "@/components/theme-provider"
 
export const Route = createRootRoute({
  head: () => ({
    // ...
  }),
  component: RootComponent,
})
 
function RootComponent() {
  return (
    <html lang="en" suppressHydrationWarning>
      <head>
        <HeadContent />
      </head>
      <body>
        <ThemeProvider defaultTheme="system" storageKey="theme">
          <Outlet />
        </ThemeProvider>
        <Scripts />
      </body>
    </html>
  )
}

Add a mode toggle

Place a mode toggle on your site to toggle between light and dark mode.

components/mode-toggle.tsx
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] 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>
  )
}