skeleton/sites/skeleton.dev/src/hooks.server.ts

import type { Handle } from '@sveltejs/kit';

export const handle: Handle = async ({ event, resolve }) => {
	let theme = '';

	const cookieTheme = event.cookies.get('theme');

	if (cookieTheme) {
		theme = cookieTheme;
	} else {
		event.cookies.set('theme', 'skeleton');
		theme = 'skeleton';
	}

	return await resolve(event, {
		transformPageChunk: ({ html }) => html.replace('data-theme=""', `data-theme="${theme}"`)
	});
};