Why can't you all ask me how to do it manually without downloading programs and it honestlyl takes just as long? Anyhoo...
http://www.themexp.org is the site I have always used to get my themes and bootscreens from.
Be careful though, because sometimes it will try and download an .exe file, which you don't want. If it comes up as trying to download an .exe, just hit cancel and try a few more times, and about the third try if not the first, it should be a zip file. Everything you need will be in the zip.
I can admit, I used to have StyleXP. I still have it but I am sick of having to e-mail the company when I switch computers and wait ages for a registration code transfer. Once I finally learned how to do it myself, it is a snap and all I have to do now is download new themes and bam, I'm set.