Hey Mik,
This was driving me crazy for weeks until I finally figured it out this morning. Don't know if you've found a solution yet, but here goes, for you and anyone else who stumbles on this. (It seems a number of people have had issues modifying the font in this widget.)
If you're using the Atahualpa theme, you have to modify the CSS in the theme's files, not the widget (which was my mistake). On your dashboard, go to Atahualpa Theme Options < Various Content Items < Add HTML/CSS inserts. At the very bottom is an option called "CSS inserts". To change the color of the date in the widget, you have to see what's going on under the hood. So, with the Upcoming Events widget activated in a sidebar, highlight the widget, right-click and "View selection source". You'll see that the date of an event is bracketed by ul and li tags and the div id is "upcoming-events".
So in CSS insert, add this:
HTML Code:
#upcoming-events ul li {
color: #ffffff;
font-weight: bold;
}
Or whatever other colors or styles or sizes you want to make the font.
For the header or titles within the widget, you can easily modify that by going to Sidebars & Widgets < Style widgets < Widget List Items. From there you can easily change the color and whatnot. Hope this helps!