Incompatible units: 'rem' and 'px'.

[Deekor]

Trying to override the $font-size-base variable to $font-size-base: 12px

is there any workaround to this in the gem?

[bombergio]

You can recalculate in rem based on calculation table i.e. here http://www.standardista.com/px-to-rem-conversion-if-root-font-size-is-16px/

So for your case $font-size-base: 12px would be $font-size-base: 0.75rem