Improve styles of Clean when using custom settings

With the new settings allowing for background-image, the content becomes unreadable if the image is a bit dark. The content should also have a background colour.

Perhaps the background colour of the content is not required whenever the background-image is not used. And when it is, should it be different than the background of the blocks, navbar, etc...?

If the background colour is always the same, then only @textColor is required but whenever background colours may vary, then the text colour should also be customised per background. Doing so will require a lot more work to target the text specifically.