in the rc.xml you posted, the theme is set in the second line. Unpack the downloaded theme to ~/.themes and set the theme in the rc.xml to whatever the directory name is. IIRC you will then need to restart openbox.
I just had a look at that file and although it is a gzipped tar ball it has the extention obt. This could be causing problems for you. Try the following
zcat -S ~/Downloads/96082-Aurora\ Leopard\ BSM.obt > foo.tar
tar xf foo.tar
Now you should have directory named "Aurora Leopard BSM" (with spaces in the name... why do people do that?). Next edit your rc.xml to replace "Clearlooks" with "Aurora Leopard BSM".
PS. I haven't used openbox in many years so I can't vouch for my instructions being 100% accurate.