Use light text color on dark colors in `style.background_gradient`

[soxofaan]

I'm working on a commit to enable light text color (white by default) on dark barckground when using style.background_gradient. This improves readability, without having to limit the color range with the low and high arguments.

I'm not completely sure yet about the argument name, which is light_text at the moment. Any suggestions?

Example:

[soxofaan]

(related to #15204/#21259)

[soxofaan]

there is already a PR under construction for this apparently: #21263