Add new colors and define green theme

[steff456]

Part of https://github.com/conda-incubator/conda-store-ui/issues/288

Description

This pull request:

• [x] Defines a green theme
• [x] Adds new objects with the colors defined in the color system
• [x] Picks the right theme depending on the preference setting

Pull request checklist

• [x] Did you test this change locally?
• [x] Did you update the documentation (if required)?
• [x] Did you add/update relevant tests for this change (if required)?

Additional information

• MUI only defines three shades per color, light, main, dark so I picked three shades in the palette, this can be iterated later on another PR if necessary
• MUI defines other roles for colors like error, info, success, etc. Do we want different colors for those roles or should I use the same accent color for all of them? This as well is something we can add in a follow up PR if necessary.

[trallard]

Note I have an unfinished review

[steff456]

To confirm, we're only defining the theme here, correct? And, we're yet to update the UI to use this theme? (The UI seems to have hardcoded colours right now). Please feel free to merge if this is accurate. :)

Yes! This is the base PR, I will update the use of the theme across the code base as I start implementing the changes for the design system per component