Dark theme support

[bogdan0083]

Add support for dark theme. Ideally we want to have three options: Light, Dark and System. System should be default and will depend on user system theme color.

[bogdan0083]

We added support for basic dark mode in #15, let's recap what else is needed to be implemented to get the issue fixed:

• Implement dark theme based on system preferences @media (prefers-color-scheme: dark)
• Add theme switch