Do you mean change other HTML-level stuff by making a single change to the <body> element?
Pretty sure if you have your "All H1 Headings" etc. set up in REM, you can make a change to the Body (All Pages) element that will cascade to the others.
For example, if you have all H1s set to 3 rem, and all H2s set to 1.5 rem, and you change your Body (All Pages) element from 16px text height to 20, your 3 rem H1 will go from 48 to 60 computed pixels and your h2s will go from 24 to 30 computed pixels.
