The Theme uses
rem( rem (root em) units) relative to the root (html) font size where
em is relative to its direct or nearest parent.
em’s were available first in browsers. rem’s added later so a lot of “older” references tend to show examples in em’s. All browsers now (iOS > 5), IE > 8) support REM as a unit of measurement.
So the answer is either would work. You can end up with very different results using
rem as they are relative measurements to different elements. Most modern designs are now using
rem because if elements direct or nearest parent change then the element might unexpectedly change.
The reality is you can use any CSS valid length unit like
px. Just remove the space.