Make section row text color themeable

[spacegaier]

Breaking change

Proposed change

A month ago we aligned the section header color to use the correct co0lor, but did not provide a way for users to adjust if required. This PR now adds a theme var.

Reg. its name: For a ha-card we do not use *-text-color but --ha-card-header-color since it affects the label and the icon. For the section row we do not support icons currently, but if we might in the future, then --section-header-color perhaps might be more open than --section-header-text-color.

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

• This PR fixes or closes issue: fixes #
• This PR is related to issue or discussion:
• Link to documentation pull request:

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