Theme colors and dark/light mode context

[acollierr17]

I added the styles for the green and purple color themes. Feel free to discuss the color codes used:
- Green: #2ecc71 Screenshot
- Purple: #a846eb (already existed) Screenshot

Also, for handling automatic light/dark mode functionality, I initially wasn't too knowledgeable on the matter; therefore, if you look at the version history of applyMode.js, my change was useless. Today, I made it clear how the theme is to handle light/dark mode if the user's browser does not support the prefers-color-scheme CSS media query. However, if it's supported, the theme sets the color scheme based on a) their operating system's configured color scheme or b) their browser's color scheme override.

Stuff I still need to account for:

• Apply color context to the sidebar's scrollbar
• Apply color context to the Pacman loading icon
• Apply color context to the loading/progress bar

[t3ned]

I'm not knowledgeable with frontend. The new theme colours look great!