Changing fonts and sizes to match theme defaults

I was hoping someone could shed a little light on how to change the fonts on the layout to match the one the user is using as the default font. I've tried adding add the following code into the custom CSS area: font-family:"helvetica neue"; but it doesn't seem to have any effect. I'd also like to find out if changing the font sizes is viable too.

Thanks a bunch in advance for the help and info. I'm looking forward to get this thing up and running.

Side-note: Amazing plugin, thanks for making it!