I am tweaking the look of Gnome Shell and was asking my self what the following, and also specifically what "ems" means in gnome-shell.css
/* The remaining popup-menu sizing is all done in ems, so that if you * override .popup-menu.font-size, everything else will scale with it. */
Would somebody explain it to me? I prefear to not break anything if possible, so thats why I am asking.
Thanks in advance! Roberto