fix(theming): unwanted variables and colours calc fixes

[skjnldsv]

Fix #37656

If the admin previously uploaded an image, it is not being deleted when the background is set to color only.
Meaning --image-background was being defined anyway
Now it is being defined but we ensure it is set to no, an invalid value, when the bg is deleted

Fix #38061

We were not passing the current main background to the elementColor function, meaning we were not benefiting from the max/min contrast. Now we do. If a value is too bright or too dark, the color will be replaced to ensure proper contrast.

[szaimen]

We were not passing the current main background to the elementColor function, meaning we were not benefiting from the max/min contrast. Now we do. If a value is too bright or too dark, the color will be replaced to ensure proper contrast.

Doesnt seem to fix the problem in my testing:

[szaimen]

If the admin previously uploaded an image, it is not being deleted when the background is set to color only.
Meaning --image-background was being defined anyway
Now it is being defined but we ensure it is set to no, an invalid value, when the bg is deleted

seems to work but when the admin uploads a background image and then clicks on delete (so that only the background color stays) the by the admin uploaded image will still be shown as standard background even though it was removed:

[skjnldsv]

the by the admin uploaded image will still be shown as standard background even though it was removed

Nice catch! Let me fix too

Doesnt seem to fix the problem in my testing:

What should I see in your screenshot?

[szaimen]

What should I see in your screenshot?

The buttons should have a more greyish color to better distinguish them from the background even if the primary color is set to black?

[skjnldsv]

What should I see in your screenshot?

The buttons should have a more greyish color to better distinguish them from the background even if the primary color is set to black?

Well, those buttons uses primary css variables that are not made for elements

That needs to be fixed in the vue components then :)

[skjnldsv]

/backport to stable26