Several sites provide free themes compatible with 4.5. I've recently downloaded and have safely used themes from... Free Blackberry Themes For 8100 8300 8800 New 4.5 Plazmic BlueRoomSolution Themes4BB.com| Free BlackBerry Themes| Multimedia For Your Berry Free BlackBerry Themes - PinStack.com - BlackBerry forums
Some of these sites require registration (which is free).
Before you download just make sure it's compatible with your BB and your OS. I've also used themes on my 8310 (188.8.131.52) that were designed for 4.2.2. The ones that I've heard cause very serious problems for 4.5 users are RIM's Today Plus and RIM's Surreal, though I'm sure there are more.
Also, I found that some 4.5 themes didn't work until I reinstalled all the BB Dimension themes.