fix(theming): Use `AppConfig` for setting background image #12058

Job	Run time
changes	6s
integration-sqlite (capabilities_features, 8.1, main)	2m 35s
integration-sqlite (collaboration_features, 8.1, main)	3m 47s
integration-sqlite (comments_features, 8.1, main)	3m 24s
integration-sqlite (dav_features, 8.1, main)	8m 17s
integration-sqlite (features, 8.1, main)	7m 10s
integration-sqlite (federation_features, 8.1, main)	4m 37s
integration-sqlite (--tags ~@large files_features, 8.1, main)	12m 9s
integration-sqlite (filesdrop_features, 8.1, main)	2m 29s
integration-sqlite (openldap_features, 8.1, main)	3m 5s
integration-sqlite (openldap_numerical_features, 8.1, main)	2m 45s
integration-sqlite (ldap_features, 8.1, main)	2m 44s
integration-sqlite (remoteapi_features, 8.1, main)	2m 35s
integration-sqlite (setup_features, 8.1, main)	2m 2s
integration-sqlite (sharees_features, 8.1, main)	3m 45s
integration-sqlite (sharing_features, 8.1, main)	14m 1s
integration-sqlite (videoverification_features, 8.1, main)	6m 45s
integration-sqlite-summary	5s
	1h 22m 21s

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