When you say that you close the apps do you actually goto close or exit from the menu? And you should really turn off your wifi/bluetooth if you're not using it, that will really save some battery life.
As for the theme problem, you have to make sure you download the theme that is made for you specific OS and device, which you can find that out from Options --> About. If you try and install a theme made for another device it won't work.
Alot of the third party apps, like facebook, need a Blackberry Data plan, and won't work on wifi alone. Which you will need to contact your carrier and have the plan activated for your account.