That is because each element in the front page has its own background colour, except for the static front page content. Changing background colour will also mean that you would need to change the text and border colours as well, which is why for now we haven’t added it yet.
Developing a proper colour customisation module is quite a major undertaking. We will be adding it in the future so that we can really develop a proper module rather than pushing for rushed code in the initial release.
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