Implement loading custom css in the GTK app #9257

Job	Run time
test	2m 33s
alejandra	32s
prettier	34s
typos	32s
build (namespace-profile-ghostty-md, aarch64-linux)	1m 3s
build (namespace-profile-ghostty-md, x86_64-linux)	59s
build-bench	58s
build (namespace-profile-ghostty-md, x86-windows-gnu)	1m 1s
build-macos	4m 44s
build-linux-libghostty	1m 1s
build (namespace-profile-ghostty-md, x86_64-windows-gnu)	1m 0s
build-macos-matrix	11m 27s
build-windows	6m 8s
build-nix (namespace-profile-ghostty-md)	2m 24s
build-nix (namespace-profile-ghostty-md-arm64)	4m 54s
GTK adwaita=true x11=true	1m 3s
GTK adwaita=true x11=false	1m 5s
test-macos	3m 20s
GTK adwaita=false x11=true	1m 4s
GTK adwaita=false x11=false	1m 4s
	47m 26s

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