Fix MessageList colors for better contrast

[wmontwe]

This removes the custom color attributes and replaces them by Material 3 colors.

In result it's already better for the light theme, while the dark theme could be still tweaked to enhance the visual distinction between read and unread emails.

[kewisch]

Let's discuss this with design first before we remove the colors, good to have a final suggestion before we proceed with intermediate changes

[wmontwe]

I removed this attributes as they should have been removed already before. They are just blocking the view on the actual issue. And yes, this has been discussed and agreed before with design. Just other things have been more important. But anyway.

The colors needs to be adjusted to follow the last design proposal.