I mentioned Gnome Shell extensions the other day. Two of the extensions supported theme management. Maybe you'd like to change the look of your Gnome desktop? Under Linux, this is a very simple thing to do. Although Gnome 3 supported themes "out of the box", including Gnome 2 themes, I'll admit that it took a little while for themes specific to Gnome 3 to appear.
The best gallery seems to be at DeviantArt.
My current favorites are Smooth Inset and Elementary.
These are some others I've tried, and really liked: Smooth Inset Small-Screen, and Adwaita-White Netbook. I'll also give a nod to Nord, which is nice, but the author advises it is beta.