I guess you are referring to the GTK/QT/... theme used? Most tiling WMs don't set one by default, but you can easily do so by using a custom .gtkrc-2.0 or so[1].
[1] see, for example, https://wiki.archlinux.org/index.php/GTK%2B#Themes
Also, I find lxappearance an easy and quick way to set GTK themes via GUI rather than having to edit the .gtkrc files for every option.