Remove hardcoded background colour in the GUI dialogs #529

Job	Run time
Build MDANSE and run tests on ubuntu-20.04	3m 27s
Build MDANSE and run tests on ubuntu-20.04	3m 20s
Build MDANSE and run tests on ubuntu-20.04	3m 22s
Build MDANSE and run tests on ubuntu-20.04	3m 57s
Build MDANSE and run tests on windows-2019	4m 36s
Build MDANSE and run tests on windows-2019	5m 16s
Build MDANSE and run tests on windows-2019	4m 14s
Build MDANSE and run tests on windows-2019	5m 2s
Build MDANSE and run tests on macOS-11	6m 44s
Build MDANSE and run tests on macOS-11	7m 16s
Build MDANSE and run tests on macOS-11	6m 41s
Build MDANSE and run tests on macOS-11	6m 45s
	1h 0m 40s

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