Little Forms is an open source (but note) minimalistic GUI toolkit written in plain C89. It currently supports Windows and X11 backends (for Linux, BSD, Mac OS X and other systems that provide the X11 API).
The goal of Little Forms is to provide a simple and cross platform C GUI toolkit with a stable API once version 1.0 is released (see API Stability). The current style is designed after Window Maker, which itself was designed after NeXTSTEP and GNUstep. A styling API might be provided in the future after the library is stable, but it is not a priority at the moment.