Pick a theme for Fluxbox that has a compatible GTK+ counterpart and apply a clean consistent look across applications.
Download Finery Dark, untar the package and place the theme folder in ~/.fluxbox/styles (create the styles folder if necessary). Right-click on the Fluxbox desktop and select Styles->Finery Dark.
Download the Finery Dark GTK matching theme, untar the package and place the theme folder in ~/.themes (create folder if necessary).
GTK themes require an engine to run. Download the murrine engine to run Finery Dark and some GUI apps (thanks Dan for sugesting gtk-chtheme) to make switching between themes easier ...
$ sudo apt-get install gtk2-engines gtk2-engines-murrine gtk-chtheme qt4-qtconfig $ gtk-chtheme
Select your newly-installed Finery Dark theme.
Apply a GTK theme to all your QT4-based applications by running qtconfig. Select the Appearance tab and modify Select GUI Style from Desktop Settings (default) to GTK+.
Done! Looking good Fluxbox (... and GTK ... and QT ...)!