When talking about the theming, we may actually need to talk about two different aspects of it, though they usually come together. There are UI “appearance” settings such as font weight and size, background color, border width, etc. But there are, also, layout templates which are meant to define common application layout, layout elements, and, sometimes, even the behavior of those elements.
There are theming components in the Creator Kit:
And there is a theming solution in the CoE Starter Kit:
How to apply theme to Fluent UI React components