How well do you know CSS? You can change the base font by adding this to the Header Script field under Webmasters options (Appearance > Theme Options, click on the Webmasters tab).
<style>
html, body, div, span, applet, object, iframe,
h1, h2, h3, h4, h5, h6, p, blockquote, pre,
a, abbr, acronym, address, big, cite, code,
del, dfn, em, font, ins, kbd, q, s, samp,
small, strike, strong, sub, sup, tt, var,
dl, dt, dd, ol, ul, li,
fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td {
font-family: tahoma,sans-serif;
}
</style>
If you use Firebug or Chrome Developer Tools, you can figure out what other selectors to use.