Add ha-subppage toolbar css styles

[dcramer]

This addresses an issue with styling the toolbar in the same way that the app-header had been previously addressed.

This specific case can be reproduced by doing the following:

• Set primary-color to a dark color (e.g. you want a dark header)
• Open the notifications box, the links are dark (because they use primary-color for the anchor tags)
• Switch primary-color to a light color, requiring you to redefine the app-header background color, and you'll find that most pages render as expected (other than HACS [already fixed upstream] and the integrations page, afaict)

[homeassistant]

Hi @dcramer,

It seems you haven't yet signed a CLA. Please do so here.

Once you do that we will be able to review and accept this pull request.

Thanks!

[dcramer]

I'm not sure this is the right approach, but I'm new to the hass codebase and mostly just opened the PR to create a discussion around the best solution.

You'll see the behavior I'm trying to address by using the 'synthwave' theme from the HACS default repos.