How can I add a new or custom font to default tailwind
You can do this by entering "tailwind.config.js" and extending the font family in the default theme, For example...

/** @type {import('tailwindcss').Config} */
module.exports = {
  content: [
    "./pages/**/*.{js,ts,jsx,tsx}",
    "./components/**/*.{js,ts,jsx,tsx}",
  ],
  theme: {
    extend:
    {
      fontFamily: {
        'brandnew': ['New-font', 'sans-serif']
      },
    },
  },
  plugins: [],
}

As well as properly importing the desired font into your "index.css" file.