tests

Clean up and update built-in themes and make them part of `markdown.tex` and `markdown.sty` #3150

Job	Run time
Style check (Line length)	56s
Style check (Lua)	53s
Style check (Markdown)	1m 12s
Style check (Python)	23s
Style check (Bash)	3s
Style check (Tabs and spaces)	2s
Type check (Python)	38s
Build Docker image (latest)	14m 33s
Test Docker image (latest)	55m 38s
Automatically merge pull request	3s
Publish Docker image	0s
Publish artefacts and create a prerelease	0s
	1h 14m 21s

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