Allow themes to have user-customizable color pickers

[imjoshin]

This PR adds the ability for theme developers to add CSS colors that are customizable by the user.

By adding any amount of "colors" to the theme:

jround: { 
  text: 'Round', 
  colors: { 
    background: {r: 0, g: 0, b: 0, a: 0.3} 
  }
},

User's will be able to modify the colors added in the "General" tab.

These variables are accessible in theme scss files by using the variable --color-theme-X, where X is the key name in the colors object.

These UI changes are only visible for themes that have custom colors.

[lgtm-com[bot]]

This pull request introduces 1 alert when merging 6f7666c3b5098d8455a7b45e477d3871a27545dc into cadaf940a108563383d1152b8289145a722693d9 - view on LGTM.com

new alerts:

• 1 for Unused variable, import, function or class

[imjoshin]

Hey @dsrkafuu ! Any thoughts on this and subsequent PR #28 ?

Thank you 😄

[imjoshin]

I'll get these merge conflicts wrapped up soon. If you have any questions feel free to let me know!

[imjoshin]

Closed in favor of #31