You can set the default font like this...its on a field by field basis however.
Add new rule with style and font you require to the correct HTML field within the Business Object (Don’t forget to put the <br> or it won't work)
<P style="font-size: 13px; font-family tahoma"></br></p>
Please note in images field names are not consistent (they should be just different systems used for shots)
I ended up using a slightly different line to set the font the way we wanted it. It seems to interfere less with the Knowledge article content we want to add.