The Preferences window contains user-controlled options for how fv behaves and looks. Click on one of the labeled tabs to bring up one of the following sets of options...
Click on the appropriate button to change the color of the background/foreground/Active background/Active foreground/Check button.
Click the Save button to permanently save your preferences. (Under UNIX it is saved at ~/.fvrc; under Windows it is at <Installed fv Root>\lib\fv\fv.ini; under Mac OS it is saved in the Preferences folder as fv Preferences.) Otherwise, any changes made affect only the current session.