Did you know that Builder has a build panel finally? That’s right, you can actually do what the name says now. Of course, we only have one IdeBuildSystem implementation at the moment (autotools), but if you implement that interface, you too can have your project building.
Note that you can write build system implementations in Python or Vala now. That should lower the barrier for contributions quite a bit.
Never mind the CSS errors in the theme, those will be fixed soon enough