The File->Configure->Fonts dialog

This dialog allows you to change the fonts used in various places in Xastir. You can use any font that is installed on your system and recognized by the X window system.

The font names in the dialog are the ones that the X window system uses, and are not really very easy to create yourself. You can use the "Xfontsel" button next to each one to bring up the "xfontsel" utility that allows you to explore available fonts and select one. Once you've found a font that you like in xfontsel, chose "select" and then "quit" and the Fonts dialog will be updated with the name of the font you chose.