Minimize Work needed for Theme Changes/Additions

[pr1sm]

Is your feature request related to a problem? Please describe.
With #344 about to merge, we have theme support and some automation with regards to adding new themes. This is focused on sass specifically and is great! However, we still have css-in-js classes that are applied to the react-select components separately from sass.

The current implementation will require manual changes to be made when adjusting or adding themes.

Describe the solution you'd like
Ideally we should have a separate file that contains all theme related style data and have both the sass classes and css-in-js styles pull from that single source of truth. This would minimize the work needed to adjust/add themes as well as keep both style injections in sync with each other.

There will be two things we need to take care of:

1. Separate out theme data to a format that both sass and js can pull from
2. Adjust the select.jsx file to lookup data rather than have it hardcoded

For #1 I think we can use a .json format to handle the theme data. This can be imported automatically in js and we can use a library to get it imported into sass (something like this library would work).

For #2 I think we can simply look up the data from the .json file based on the given theme that is passed in.

Describe alternatives you've considered
There are other libraries that allow you to extract structured variables from a sass file, but I'm not sure which direction is more efficient.

Additional context
Originally from https://github.com/walmat/nebula/pull/344#discussion_r258280697