chore: delete some unused files

postcss.config.cjs

-module.exports = {
-  plugins: {
-    tailwindcss: {},
-    autoprefixer: {}
-  }
-};

src/hooks.client.ts

-document.documentElement.classList.toggle(
-  'dark',
-  localStorage.theme === 'dark' || (!('theme' in localStorage) && window.matchMedia('(prefers-color-scheme: dark)').matches)
-)