gtksourceview: apply selection color from theme #749

Job	Run time
htmldocs	25m 38s
rip-and-test	18m 5s
package-arch (debian:buster)	21m 53s
package-indep (debian:buster)	48m 19s
rip-and-test-clang	18m 10s
package-arch (debian:bullseye)	22m 27s
package-indep (debian:bullseye)	47m 52s
package-arch (debian:bookworm)	24m 32s
package-indep (debian:bookworm)	43m 8s
package-arch (debian:sid)	24m 34s
package-indep (debian:sid)	43m 14s
	5h 37m 52s

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