To override the font-family and font-size and line-height in any Moodle theme you need to address these in #page rather than body.
#page {
font-family: Tahoma, Helvetica, Arial, sans-serif;
font-size: 108%;
line-height: 1.4;
}
Also...and I think you already know about Theme Designer Mode so I donlt need to say it again.
Cheers
Mary