A recent project used the Google Calendar widget - this is embedded in a iframe (in a standard Drupal block) and thus can't be styled with CSS as per normal - but we wanted to style it to match the overall design.

After a bit of hunting around for a solution I found this: MyGoogleCal. The download link is a bit hidden - grab it directly here...

Unzip it and drop it in your site somewhere, I popped it in my sites/all/libraries folder, then edit the iframe src URL. Check out the full install instructions.

Worked a charm, check it out.