fix(theming): Adjust theming util to calculate primary element color based on WCAG color contrast

[susnux]

Summary

This adjusts the theming utils for calculating the primary color:

1. Adjust luma calculation to match with the algorithm used by WCAG 2
2. Add function for calculating color contrast according to WCAG 2
3. Fix the invertTextColor by not simply checking the luma channel of the HSL color but the real color contrast
4. Improved primary element color calculation by providing the background color so an element color with high enough contrast can be calculated

Also keep blurry background and hover states in mind while calculating the element color.

Moreover I added a unit test for accessible colors.

Screenshots

Only the hover state has changed:

before	after

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[susnux]

/backport to stable28

[susnux]

Had to skip the test if the base class was run as a test case on its own as - of cause - the $theme was not set there. So in that case the test need to be skipped and not failed.

[ShGKme]

Is it intended change?

Dark theme:

Before	After

[susnux]

Is it intended change?

Yes and no.
Yes as the primary-element-text color is adjusted for color contrast.
No as this is caused by using the wrong color variables, the menu is shown on the primary color, thus color-primary-text should be used and not color-primary-element-text.