I am interested in establishing default font sizes for the different aspects of my surveys. I see that I will need to edit the css files in a template. My question is what are the locations in the file that I can find where to change everything?

I am also interested in this. Changing font properties (e.g., size and color) inside questions is possible via html commands, but its quite cumbersome and inefficient.

This depends entirely on the template being used.

I suggest using browser developer tools (F12 in most browsers) to explore the elements and associated style rules.