The PGlabs are pretty complicated under the hood, and they do have to adjust some CSS to overcome issues in the WeBWorK themes. The copy that is linked to the PGLab button on the editor is older than the one we are running for this course, and changes to the math4 theme have not been taken into account. That code should be updated, but I don't have access to the course to do that. I'll look into getting it fixed.
In the meanwhile, this version is a little more up-to-date (though still not perfect). You cold bookmark that link.