[FEATURE] Add themes

[anf]

Add themes for more customizability.

Currently planned themes:

• Dark theme (currently being used)
• Light theme
• Solarized/Monkai theme

[anf]

Themes have been added, more themes shall be suggested here, I'll add 2 more themes and an option for a custom one.

[anf]

A high contrast theme

[anf]

Customizable theme