Bright color fallback uses weird button style

[nickvergessen]

1. Use F1F1F1 as a primary color
2. Go to any page that has a primary button

3. Hover it

I think on 2 either the button should be lighter, or the font white
and on 3 the button should not hover to blue, but either to a lighter or darker version of the new button color.

[nextcloud-bot]

GitMate.io thinks possibly related issues are #4902 (Download button on share view has strange color), #1747 (Contacts App generated colors are too bright), #3203 (Shadow for buttons looks strange and off-style), #6367 (Modernize style of 'Alternative Logins' buttons), and #7105 (Weblogin flow: login button shortly wrong color).