Force light theme for generated documentation #203

Job	Run time
build-autoconf (macos-15)	4m 44s
build-cmake-qt-gist	6m 13s
build-cmake-windows (shared, ON, OFF)	23m 55s
build-autoconf (ubuntu-latest)	14m 24s
build-cmake-unix (ubuntu-latest, ON)	19m 25s
build-cmake-windows (static, OFF, ON)	22m 40s
tidy	10s
build-cmake-windows-msys2	9m 9s
build-cmake-windows-vcpkg	26m 3s
build-autoconf-windows	17m 10s
build-cmake-unix (macos-15, OFF)	8m 6s
build-32-bit-linkage	41s
build-cmake-unix-ninja	13m 10s
build-cmake-qt5-gist-smoke	5m 52s
	2h 51m 42s

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