From B2evolution

a_noskin.php is a file found in your base directory (the /blogs directory in the installation package) which has a very specific use. It is provided as a way to list the output of your blog without including the layout and styles of a skin. This will produce a very plain looking page with your blog posts, comments and other content elements, which can then be used as the user sees fit.

If you open up a_noskin.php in a text editor each section of the file is commented upon to let you know how to configure the file correctly. As the file comments say, "You only need to use this file for advanced use/customization of b2evolution. Most of the time, calling your blog through index.php with a skin will be enough. You should try to customize a skin before trying to use this fle."

Note the line:

 $skin = '';

which is what effectively disabled the use of a skin.