More and more GTK themes with strongly flat tonnes appear lately as the minimalist aesthetics design trend/wave keeps on gaining ground generally. While doing everything flat sounds simpler than designing realistic elements, it needs to be done correctly, or it may look poor, anachronistic and obsolete.
Thankfully, many artists do it right like the Serbian nale12 who among other flat things, has created Flattastic which is a set of colourful flat GTK 3.10 themes that will completely change the feeling you get from interacting with your desktop environment.
Flattastic comes in eight colours and the choice to have light or darker elements. You can setup any colour you want, by just opening file settings.ini and changing the value of ‘selected_bg_color’
The theme works well with GTK 2 applications too, but note that if you are using Qt tools often you should use a light Flattastic theme because of related text colour issues. Also, you should put the theme files under “/usr/share/themes” as the “~/.themes” directory caused problems for me.