You can either do it at the time of variable definition by specifying the font for that variable like:
DEFINE VARIABLE xyz AS CHAR NO-UNDO FONT <Specify a font for the variable>.
OR you can also define it at the time of FORM and FRAME definition, like for which field in that form and frame have some different Fonts by puttinf font and it's expression in front of that field in the form or frame. Like
FORM
v-abc AT 5 FONT <Specify a font for the variable>
v-zzz AT 25
with frame f-chk1.
hope it will help. BTW, whenevr you ask any query just mention which version of progress you are using and you need to do that change in CHUI or GUI.