cleanup: Rename all .css files to .qss. #1690

Job	Run time
Verify release asset signatures	4s
Validate PR	2s
Check for translatable strings	0s
Update nightly release tag	4s
Alpine (static)	0s
Debian	0s
Alpine	0s
Clang-Tidy	0s
Fedora with ASAN	0s
macOS user	0s
Ubuntu LTS	0s
AppImage on alpine (full, Release)	3m 23s
Docs	1m 53s
Flatpak	4m 20s
Android (armeabi-v7a, Debug, 6.2.4)	3m 34s
Android (armeabi-v7a, Debug, 6.8.1)	3m 37s
macOS distributable (arm64)	2m 49s
macOS distributable (x86_64)	4m 6s
Android (armeabi-v7a, Release, 6.2.4)	3m 15s
Windows (i686, Debug)	4m 52s
Android (armeabi-v7a, Release, 6.8.1)	3m 26s
Windows (i686, Release)	4m 36s
Android (arm64-v8a, Debug, 6.2.4)	3m 29s
Windows (x86_64, Debug)	5m 46s
Android (arm64-v8a, Debug, 6.8.1)	3m 45s
Windows (x86_64, Release)	5m 15s
Android (arm64-v8a, Release, 6.2.4)	3m 6s
Android (arm64-v8a, Release, 6.8.1)	3m 28s
Test AppImage	22s
Test macOS distributable (arm64)	10s
Test macOS distributable (x86_64)	16s
Test Flatpak	49s
	1h 6m 27s

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