Hi,
Did a quick google search and found this article:
Search Results Font
Someone mentioned that it's possible through CSS.
Google Custom Search AJAX Decoded -
Using CSS Customization
Please give it a try and let us know if you managed to change the font.
Regards,
/kksou
p.s. Please note that your browser cache the CSS style sheets. So when you change the .css styles, make sure you clear the cache so that the style sheets are refreshed. Otherwise it will still be using the old cached version.
One quick way I found that make sure that the browser is using the new modified stylesheet is to load the .css file direct in your browser (by using the URL to your .css file). If it reflects the old cached content, just click reload until the content is refreshed. You will then be sure that it's using the new modified one.