[Backport release-3_38] [themes] Fix bad styling of scrollbars in the plugin manager dialog #33085

Job	Run time
build (22.04, 5, true, OFF, ON, ON, ON, ON, OFF, OFF, OFF, OFF, ON, false)	17m 16s
build (39, 6, true, ON, OFF, ON, ON, OFF, OFF, ON, ON, ON, OFF, false)	29m 4s
clang-tidy (22.04, 5)	5m 39s
Run tests (5, ALL_BUT_PROVIDERS)	32m 1s
Run tests (5, POSTGRES)	6m 29s
Run tests (5, HANA)	6m 46s
Run tests (6, ALL_BUT_PROVIDERS)	30m 52s
Run tests (5, ORACLE, 22.04, binary-for-oracle)	7m 36s
	2h 15m 43s

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