while if you want to theme it you need to root
http://forum.ppcgeeks.com/htc-evo-4g-themes-mods/124205-sgbz-evo-themes-clear-lakers-pynk-more-froyo.html
this link has hello kitty
as for the best apps that depends what kinda apps you want
games, utilizes, etc etc
as for changing the clock there is alot of clock widgets in the market
i dont know which one is the best so download one and give it a try see if you like it
as for the battery life the new ota is said to give better battery life
there are apps in the market that can help with battery life like juice defender and screebl