Ok, it's not that obvious to configure X applications, they don't have menus, dialog boxes like the KDE applications. You have to edit ~/.Xresources and add some lines and then run xrdb ~/.Xresources (you can use -merge to merge the settings). You can find out what lines to add to ~/.Xresources by having a look at /user/X11R6/lib/X11/app-defaults/XTerm and by consulting the manual page (man xterm).
Here is what my ~/.Xresources file contains:
xterm*background: blackI usually use konsole, but some applications open xterm for shell access. SlickEdit is one of those applications ;)
xterm*faceName: Andale Mono