I used Flat Awesome + three years ago for Xenforo 1.5 and everything was comfortable and worked fine. (y) In particular, font settings:
But the new version, Xenforo 2.1, I can not understand... :( Many settings don't work out the way I expect. I have a few problems.
For test I chose monospace for Body Text Font.
Problem 1. On the resource page, the content is used User Interface Font Exo 2. This is wrong.
Problem 2. In the forum topics everything is fine. Font used Body Text Font monospace, which I have specified:
It is right. BUT!!! When typing in a text editor is used User Interface Font Exo 2. This is wrong. ?
P.S. Sorry for my bad English.
So may actually be an oversight on XenForo actually, that Body Text Font is only used thread messages themselves, not resource messages. You can add this to your extra.less:

.resourceBody, .fr-element { font-family: @xf-fontFamilyBody; }

Should adjust the resources and the editor, I'll look at adding it for a future release.
You can remove the above CSS as of 2.1.1, it's fixed officially inside XF2.1.1 (following up with my internal list of bug reports :) )

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