Unrelated to #gamedev, I learned a bit about infographics today as well as Clip Studio EX. It's been productive!
Godot Themes are simpler than I expected. They're just resources you can add and customize.
When you access a new theme, you can see a Theme preview as well as an Inspector with few properties.
To customize your theme, you can use the Edit Theme menu. With it, you may add new items (which represent UI controls) or just start with a template. The items show up in the Inspector.
You can add sub-resources or reuse file resources to set different item properties and get a desired custom result. I know I'll be tinkering with this for a bit!