Right-click over a text window and select "Filetype settings->Display and fonts" in the pop-up menu. The actual name of the menu item may be different, I'm used to the German version.
Or: Select "Global settings" in the menu and go to the filetypes page. You will find a "Display and fonts" item in the properties of the standard filetype, doubleclick on it.
Some filetypes have their own display setup and do not share the standard display; in that case, unfold the properties of the filetype and doubleclick on the "Display and fonts" item there.
