I generally made my own icon-theme and took icons from sets which I like as I only like the huminaty app icons better but not the rest of the humanity icons. I like the simple fullscreen button from hicolor better than the fullscreen button from humanity.
So I made an icon-them myself. To start, I took the Pleego-Remix-icontheme as base, which only has some apps icons and no other system icons. Then started adding icons of apps I liked in the /opt/usr/share/icons/pleego-remix-icons/64x64/48x48/apps and/or /opt/usr/share/icons/pleego-remix-icons/64x64/apps folder.
Ofcourse you need to look up the name of the apps icon in the .desktop file or the matching name in hicolor.
Some of the icons are from Hicolor, some of them from humanity and some just from the internet.
Some reasons: the aformentioned fullscreenbutton
In humanity-icon-theme I think conversations looks too much like e-mail.
Some Hicolor are just too flat for my taste.

PS. If you like my pictures, feel free to take a look at my flickr (in sig)