Isn't this from tailwind.config.ts in the root of the project?

This come from global.css (imported in layout.tsx) added by create next app.

I think they already deleted that part of global.css

If you create your app with create-next-appthen the stuff below gets added to tailwind.conf.ts:

const config: Config = {
  content: [
    './src/pages/**/*.{js,ts,jsx,tsx,mdx}',
    './src/components/**/*.{js,ts,jsx,tsx,mdx}',
    './src/app/**/*.{js,ts,jsx,tsx,mdx}',
  ],
  theme: {
    extend: {
      backgroundImage: {
        'gradient-radial': 'radial-gradient(var(--tw-gradient-stops))',
        'gradient-conic':
          'conic-gradient(from 180deg at 50% 50%, var(--tw-gradient-stops))',
      },
    },
  },
  plugins: [],
}
export default config

so you can just delete the "theme" part to get rid of it