I presume one way is to go through all the style properties (ie General and Message Elements) plus any addon style properties and change the fonts in each one.
But it occurred to me that it might be better to just remove all those fonts, and just leave the font I want in the General> Body.
This seems to work fine, and allows me to change the font of the forum globally.
Is this the best way? Any reason why I should not do this?