Replace 'buttontheme = oxygen' with Numix set

On line 7, it would be nice to change 'oxygen' with the name of the numix square icon set. I wouldn't be so sure what the name of the icon set is in the case of oblogout, but I have tried a few names without luck. I take it you would know the name, or the directory the icon set might need to be copied to for the icons to work?