I guess that still has the problem that we have no idea what GTK+ theme the user will choose, so it's not really possible to pick properly contrasting colours.
Its true, but we can pick bright colours instead of the dark ones currently implemented. Its very likely that it will give a contrast that is acceptable to use with dark themes (if it is beautiful or not, I don't judge). As a last resort, the user can still edit the CSS file. To complicate a little more, we can mount these ready-to-use themes to give more options too. If one isn't suitable, comment it and uncomment the next in the CSS file.