This dialog allows you to modify the font OmegaT uses in the following windows:
It is called via the Options → Font... item in the Main menu.
The dialog contains:
Warning! It may take a noticeable amount of time for OmegaT to update the display after you have changed the font setting, especially when a big file with many segments is open in the editor. Also, some fonts behave better for some language pairs than for others.