Define proper colours for the installer/maintenance tool in the dark mode

The installer and maintenance tool colours should be improved in the dark mode in macOS.

• Green links are not very well visible in the dark background
• Disabled checkbox text (Qt Account login page: I accept the service terms..) is difficult to see
• The black text in the dark grey background in the setup page is almost impossible to read. 

A study about the most usable colours + a minimal implementation change is required.