Add !global on mobile variable redefinitions

Sass 3.3.0+ issues a few warnings like this:

   DEPRECATION WARNING on line 477 of style/main.scss:
   Assigning to global variable "$field-width" by default is deprecated.
   In future versions of Sass, this will create a new local variable.
   If you want to assign to the global variable, use "$field-width: 280px !global" instead.
   Note that this will be incompatible with Sass 3.2.

Adding !global as suggested works fine, with no warnings, and doesn't
change the resulting main.css at all.