Ensure visibility of buttons and radio/checkbox for dark primary color in dark theme just like we do for light primary color in default light theme

[jancborchardt]

As a follow-up to #2622 by @skjnldsv. We need a fallback for the case of dark primary color in dark theme just like we do for bright primary color in light theme.

Light theme + bright primary colour

This looks good – here we offer a fallback to a grey tone so there is enough contrast with the background, and the primary button looks more present than the secondary one:

Dark theme + black primary colour

Here the radio buttons and checkboxes are barely visible, and the primary button is way less visible than the secondary one:

[skjnldsv]

Thank you for opening this, it was on my todo!
Do you have an idea on an already existing variable we should use for the icon colour then?

[jancborchardt]

The only variable which comes close is text-maxcontrast but that would be misusing it.

I guess we need color-primary-element-dark in addition? Or can color-primary-element know if it needs to be triggered for dark themes?

[skjnldsv]

I guess we need color-primary-element-dark in addition? Or can color-primary-element know if it needs to be triggered for dark themes?

It could. We just have to define clear expectations so I know what I can put :)

[susnux]

Fixed by @szaimen in #4067

On this screen recording the primary color was set to black:

vokoscreenNG-2023-06-26_15-59-01.mp4