Theming: defaults and custom

[aayushmau5]

The current theme looks good, but we want to provide more control over theming as well as provide some more default themes.

I propose:

• 3 default themes: Dark(for dark mode), Light(for light mode) & Neon(The current one). We need to discuss what dark and light theme should look like, and that might have a long conversation, so I'm keeping this proposal as optional.
• Custom CSS: We provide the ability to add custom CSS for theming. Perhaps as a separate section, where user puts in their custom CSS and that CSS will be used for theming.

We got to have a long discussion on both of these proposals if someone is interested on implementing them. Till then, this issue is sufficient to outline the new feature request.