Your plugin has been great for some time now but recently has become a little problematic.
a) I cant save anymore. Hitting the save button now causes a page not found error. this has only recently started happening and is not browser specific. I have to ftp in and edit the stylesheet directly. It is the same problem on all sites I do now.
b) I do responsive designs so dont need a separate stylesheet for mobile. Problem is, if I turn the mobile stylesheet off, the mobiles see no stylesheet at all (I would think they should revert to the main default one). Therefore I have to leave the mobile sheet ON, and with copy and pasting duplicate the entire standard stylesheet into the mobile one. so with every site I do, I have to have the same stylesheet twice.
In the settings maybe consider a third option - turn mobile stylesheet off so that mobiles use standard stylesheet.
Due to the above, I find myself looking for another solution and if there isnt one (there doesnt seem to be) I might have to consider putting together my own plugin which goes back to the simple days - just one stylesheet for the lot so that I can put responsive styling in it. I would rather continue using yours, hence logging this!