I don't think that you can. It may be fixed in a later build or it may not. It is nice that they added it but it I'd a little bit of a poor attempt in this build.
Posted from my CrackBerry at wapforums.crackberry.com
I war really hoping the today they would have gone the extra step and had that theme encompass ALL calendars on the device instead of only showing the default one.
Either that or enable color coding of appointments in the calendar