Fix contrast in white-label header

[tzi]

Description

Header background on login page

Previously, the header was not customized in the login page. It's fixed now.

Desktop and mobile menu

When a user set up a custom header font color, the menu could have a very low contrast.

Here, you can see the issue in action and the rendering after the fix. The fix do not provides a clear focus style, but it is at least readable.

what	before	after
customized desktop
customized mobile

The rendering without any customization still looks fine.

what	before	after
default desktop
default mobile

Dark mode is impacted, the focus styles changed a bit…

what	before	after
darkmode desktop
darkmode +customized

Checklist

• [NA] I have understood, reviewed, and tested all AI outputs before use
• [NA] All AI instructions respect security, IP, and privacy rules

Review

• Functional review done
• Potential edge cases thought about (behaviour of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• New documentation added or existing documentation updated

[chippison]

This is all good and we will cleanup the screenshots and red bubble on a separate issue

Issues noted and will be worked on spearate PR's