The default is Rubik and if the browser doesn't support it, it would switch to Helvetica. If it still doesn't work on your browser, it would change to Arial. Finally, if all three don't work, it would let the browser pick whatever font from Sans-serif family
"Done. I fixed it." Good for you. But what was the problem - since you only asked for what font is used on FandomDesktop and everybody gave you the correct answer?