I can easily change my Google apps email theme to match my company's colors.
Is there a way that I can accomplish the same thing for the calendar? I looked in all the settings and so on, but can't seem to find anything interesting.
This might be because the calendars themselves use color-coding. More likely it's just been overlooked.
You can use an image as the background for your calendar. There's no reason you couldn't, say, use a large version of your company logo.