Determine icons theme automatically. #227

Job	Run time
build (ubuntu-latest, 27.2, false)	1m 21s
build (ubuntu-latest, 28.2, false)	1m 56s
build (ubuntu-latest, 29.1, false)	1m 45s
build (macos-latest, 27.2, false)	5m 7s
build (macos-latest, 28.2, false)	6m 28s
build (macos-latest, 29.1, false)	6m 25s
build (windows-latest, 27.2, false)	2m 2s
build (windows-latest, 28.2, false)	2m 35s
build (windows-latest, 29.1, false)	2m 15s
build (ubuntu-latest, snapshot, true)	1m 31s
build (macos-latest, snapshot, true)	3m 11s
build (windows-latest, snapshot, true)	59s
	35m 35s

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