RFC: refactor theme-handling internals #1112

Job	Run time
Test minimal requirements	8s
Python 3.10 on ubuntu-latest (free-threading: false)	11s
Python 3.11 on ubuntu-latest (free-threading: false)	12s
Python 3.12 on ubuntu-latest (free-threading: false)	8s
Python 3.13 on ubuntu-latest (free-threading: false)	50s
Python 3.10 on macos-latest (free-threading: false)	38s
Python 3.11 on macos-latest (free-threading: false)	12s
Python 3.12 on macos-latest (free-threading: false)	12s
Python 3.13 on macos-latest (free-threading: false)	12s
docs	10s
Python 3.10 on windows-latest (free-threading: false)	35s
Python 3.11 on windows-latest (free-threading: false)	35s
Python 3.12 on windows-latest (free-threading: false)	24s
Python 3.13 on windows-latest (free-threading: false)	1m 31s
Python 3.13 on ubuntu-latest (free-threading: true)	1m 45s
type check (3.10)	8s
type check (3.13)	9s
	8m 0s

Provide feedback