Like the default Ubuntu GTK themes? Well, there’s now a porting of Ubuntu Themes for Fluxbox which is called Fluxbox Light Themes.
See the screenshots:
Install the theme:
Directly download and install the .deb package from the launchpad page. The package was built for Ubuntu 14.04, but it also works on Ubuntu 13.10 and Ubuntu 12.04.
Once installed, remember to update these settings in your ~/.fluxbox/init:
session.*.titlebar.left: Close Minimize Maximize