tabler/preview/assets/js/demo-theme.js

/**
 * demo-theme is specifically loaded right after the body and not deferred
 * to ensure we switch to the chosen dark/light theme as fast as possible.
 * This will prevent any flashes of the light theme (default) before switching.
 */

const themeStorageKey = "tablerTheme"
const defaultTheme = "light"
let selectedTheme

// https://stackoverflow.com/a/901144
const params = new Proxy(new URLSearchParams(window.location.search), {
	get: (searchParams, prop) => searchParams.get(prop),
})

if (!!params.theme) {
	localStorage.setItem(themeStorageKey, params.theme)
	selectedTheme = params.theme
} else {
	const storedTheme = localStorage.getItem(themeStorageKey)
	selectedTheme = storedTheme ? storedTheme : defaultTheme
}

if (selectedTheme === 'dark') {
	document.body.setAttribute("data-bs-theme", selectedTheme)
} else {
	document.body.removeAttribute("data-bs-theme")
}