Themes depend upon what desktop manager you are running. I always liked getting themes and eye candy at
http://www.kde-look.org/ for KDE desktop, and I liked running superkaramba plugins, e.g. liquidweather. Right now I am running XFCE as a destop manager and have just tweaked it here and there. It does not have as much eye candy but is quicker.
playback for divx is native now for YDL 5.02 base installation. you can also run divx in mplayer or vlc player or numerous other players. there's lots of information, use search.