Find a font you'd like to use and click on Select this style
On the top right:
At the top of your style sheet paste the @import code (CTRL+V).
Select and copy what's in the CSS rules box...
Then add a new line inside body and paste it (TAB it in).
Save your style sheet and see how it looks on one of your topic pages...
If the side panel is closed, open it using this button on the top right...
You can try out as many fonts as you like. For each font you try:
Copy the font-family code for that font only...
...and paste/replace it in the body style.
If you don't like it, you just need to copy the font-family in the body style. You can also CTRL+Z until you have that font-family again.
If you like your font (or you're not sure) but it's too small, you can make the default font size bigger in body style. It's best to use em so it's just a bigger version of that font (1.5em = 150% of regular size)
In my example, I found that too big, so I reduced it to 1.2em