In the folder where you installed FileBox eXtender, there's a sub-folder named ICONS. Inside that are several standard format Windows icon files. (These have the extension .ICO.)

v      Copy a pair of these files into the main program folder, and then rename them to FAVORITE.ICO and RECENT.ICO.

v      Now exit and restart FileBox eXtender. (In some cases you will have to reboot your machine---exiting and restarting the program may not be enough to effect the change.)

v      The new icons will then appear.

You can edit these ICO files if you want, to make them look how you'd like. You'll need an icon editing program for that.