Use primary colour for luminance calculations if background is disabled

[skjnldsv]

Well, how can we know if the background you selected has a bright top header? We're gonna improve for 26, but until then, it's not a bug, just not implemented.

@skjnldsv At least you could know if one has selected like #ffffff in the color picker. Not talking about background image. In fact any picked color tending towards white makes the GUI unusable right now. Call it a bug or broken by design. It is an issue. As long as you cannot set it dynamically, a small toggle switch (switch between light and dark header icons) could also do the trick. Quick and easy.

Originally posted by @HannesJo0139 in #34965 (comment)

[skjnldsv]

26 fix in #35666
Will have to adjust for 25 as lots have improved for 26