Make themes more extendable by using JSON

[jsoconno]

Is your feature request related to a problem? Please describe.
No. Themes are currently working as expected.

Describe the solution you'd like
It would be interesting if instead of creating separate classes for each theme that we use JSON for configuration and ship those files with the architectures library. This might make it easier for others to extend themes or use custom themes for their company or purpose without needing to modify architectures directly. That means we also need to add a Custom theme class that takes a path to a JSON file for any user provided themes.

Describe alternatives you've considered
We could just have users contribute new themes directly to architectures, but it seems like that is more difficult and is not as extendable for users who want to locally adjust their theme settings to meet organizational or project requirements.

Additional context
None.