I had the same problem after switching to Mathematica 10. The issue here is the following:
Export uses Rasterize to create the png image. The StyleEnvironement, which is used in Rasterized, cannot be specified as an option but is given by the $FrontEnd object (not by the EvaluatingNotebook[]!). You can change the StyleEnvironement by
SetOptions[$FrontEnd, ScreenStyleEnvironment -> "Printout"]
When you use Export or Rasterize now, you will obtain the "Printout" style.
Note: This has been tested on Mathematica 10, Win7, 64bit. Other versions (Mac) might behave differently (see comments below). Consider using the $FrontEndSession object, as suggested by Michael E2 below.
Related questions on Stackexchange:
How to Rasterize in "Printout" style
http://stackoverflow.com/questions/6093559/how-to-export-graphics-in-working-style-environment-rather-than-printouthttps://stackoverflow.com/questions/6093559/how-to-export-graphics-in-working-style-environment-rather-than-printout )