An app in BB app world called 'detoday' for the calendar events. I have heard that os6 Torch 9800's have this feature already but I never found it on mine.
Your os6 may have it too.
I'm not sure about the missed calls.
*ignore the os6 parts. I saw you have a 9700 and misread your 9900 part. The rest still counts.
There are many apps that will be able to help you out.
Just do a search for today screen on app world.
The one I'm currently using is called Fancy Widgets
Its very good, but does cost money and does more than just add a today screen (calendar). I'm sure there are free or less expensive apps that will just put your calendar today screen on your homescreen.
There isn't an official theme builder yet. There are some themes but they are based on the theme that came with your bb and don't have the capability you are looking for yet. Definitely look in AppWorld. I liked Fancy Widgets but some functions slowed my bb down a little bit. Hope you find something that meets your needs.