Fix for Incompatible units: 'rem' and 'px'

When setting a size for the switch heights, the defaults are set in rem and comparing it to px is causing a syntax error. So I switched it to use the rem-calc() mixin to resolve the error.
Merge pull request #5391 from jameswlane/patch-2

Fix for Incompatible units: 'rem' and 'px'