Dark mode header color

[maykar]

Proposed change

This changes header background color in dark mode to use a darkened version of the primary color. This will help differentiate it from cards, especially in panel mode as right now they are the same color.

This is related to a conversation from this PR #7465 and is proposed as an alternate solution.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

Additional information

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

[bramkragten]

OK to merge after comments!

[KTibow]

Have some questions about it:

• Will we be able to override it with app-header-background-color?
• (unrelated) Can we specify a custom theme as a dark theme, and base it off of the dark theme defaults using dark?