I work with Volusion all the time and this is what I do when I absolutely can't find a way to change what I need to (only as a last resort!!). Specifically this will target all of those side menu links:
#lnk0, #lnk1, #lnk2, #lnk3, #lnk4, #lnk5, #lnk6, #lnk7, #lnk8, #lnk9, #lnk10, #lnk11, #lnk12, #lnk13, #lnk14, #lnk15, #lnk16, #lnk17, #lnk18, #lnk19, #lnk20, #lnk21, #lnk22, #lnk23, #lnk24, #lnk25, #lnk26, #lnk27, #lnk28, #lnk29, #lnk30, #lnk31, #lnk32, #lnk33, #lnk34, #lnk35, #lnk36, #lnk37, #lnk38, #lnk39, #lnk40, #lnk41, #lnk42, #lnk43, #lnk44, #lnk45, #lnk46 {
font-family: tahoma!important;
(Of course, change the font to whatever you wanted it to be). With volusion, because code is nested within tables within tables within tables (gag!), you'll need to be specific about what you are targeting.
Using !important can sometimes make css really messy but unfortunately I have to use it alot with volusion. Unlike what sweetamylase said, this trick will overwrite most of your inline style needs.
Here is an article on !important http://webdesign.about.com/od/css/f/blcssfaqimportn.htm