Provide better dark mode support

[rawilk]

This PR introduces a Tailwind plugin which will provide better dark mode support than the dark-mode.css file currently does. It will work for the following kinds of tailwind configs for the darkMode setting:

module.exports = {
    darkMode: 'media',
};

module.exports = {
    darkMode: 'class',
};

module.exports = {
    darkMode: ['class', '.is-dark'],
};

The dark-mode.css class is getting removed in this PR, and any other references to dark mode in any css file will also be removed. Essentially, dark mode will be opt-in.