If you are interested in creating custom themes definitions for Drafts, this forum category has been created as a place to get help and ask questions.
The themes article in the User Guide is the best starting point to understand what themes are, what they are for, and how to go about creating them:
Once you get through that, export several of the built-in themes as files, and open them in a JSON-friendly editor. There are also a growing number of example themes in the Directory, see Themes. After installing a custom theme, it can be exported and viewed in a JSON-friendly editor.