Adam Taylor has released Helvetical, a work-in-progress userstyle for Google Calendar, based on my Helvetireader theme:
I know I am not alone in my love/hate relationship for Google’s apps, the functionality is brilliant but the UI is ugly. Google Calendar is most definitely not the exception to the rule, it hurts my eyes. Helvetical extends the work done by Jon Hicks on Helvetireader, turning the mayhem of the Google interface into something that doesn’t offend. It was created for use with Fluid.app but I’m sure you can hack the CSS to work for your needs.
At one point, I had in mind that I would create a minimal Helvetica theme that would work across various google apps. Then I took one look under the hood of GMail and new it would never work.