You will have to edit the default\css\index.css file and change the font setting line.
On 1.1.8 it looks like
font: 87.5%/150% "Segoe UI", -apple-system, BlinkMacSystemFont, "Roboto", "Oxygen", "Ubuntu", "Cantarell", "Droid Sans", "Helvetica Neue", "Trebuchet MS", Arial, sans-serif;
which basically is apple, windows, unix, android fonts and then fall back values. We try to use the default os/system fonts. The browser will go in order in that font line and use the first one it finds installed. If you wanted to change windows from "Segoe UI" just change that to the name you want, like "Times New Roman" or "Verdana".
If you want to use a web font and not a "built in" one, you will need to include the stylesheet call in index.template.css