How can I change the font and background Colors?
Use the context menu (right-clik) for each window. It also works for the main log window.
How can I change treshold for off-time calculation?
Open the rate Window (ALT-R), open the context menu (right-click on the rate window), there is a menu point "offtime -> calculation treshhold". The treshold can be set to 10,20,30,40,50,60 or a custom value between 1 and 120 minutes.