To change TaskPaper’s font you need to modify the stylesheet that you are using. The basic steps are:
TaskPaper > Window > StyleSheet > Open StyleSheet Folder
Locate the stylesheet file that you are using from that folder. Default is
Light.less. Open that file in a plain text editor.
Paste this text (modified to have name of font you want) into that file and save:
The new font should now be applied in TaskPaper.
The easiest way to change font size is through TaskPaper > View > Zoom In and Zoom Out.
Many more stylesheet options are possible, this topic is just for those who want to change the font and be done with it!