Backport: Theme JSON Tests: Refactor base styles to a static variable #5338

Job	Run time
Core running from build / Core running from build / Linux	1m 59s
Core running from src / Core running from src / Linux	1m 52s
Core running from build / Core running from build / Windows	4m 22s
Core running from src / Core running from src / Windows	4m 10s
Core running from build / Core running from build / MacOS	1m 40s
Core running from src / Core running from src / MacOS	1m 39s
Gutenberg running from build / Gutenberg running from build / Linux	9m 20s
Gutenberg running from src / Gutenberg running from src / Linux	8m 44s
Gutenberg running from build / Gutenberg running from build / Windows	1m 9s
Gutenberg running from src / Gutenberg running from src / Windows	12m 30s
Gutenberg running from build / Gutenberg running from build / MacOS	9m 11s
Gutenberg running from src / Gutenberg running from src / MacOS	8m 54s
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