I have the same problem , did you find the solution?
this message it is apper :
Consider using ‘dppx’ units instead of ‘dpi’, as in CSS ‘dpi’ means dots-per-CSS-inch, not dots-per-physical-inch, so does not correspond to the actual ‘dpi’ of a screen. In media query expression: print, not all, (-webkit-min-device-pixel-ratio: 1.25), (min-resolution: 120dpi) admin.php?page=an_gradebook_page:1
[ No bumping please. ]
