How to change the default theme

[GOBERT Geoffroy]

    Hello all,
    First of all, i don't know why, but my XFCE have been totally 
changed ! I have some icons on my desktop, my wallpaper have been 
changed, etc etc... Is it because i've just installed nautilus ? Or 
is-it  because i've typed "apt-get upgrade" Debian GNU/Linux and so, it 
has installed XFCE 4.2.0 ?

    Next, i'd like to know how i can change the default theme of XFCE. 
Because i like my new wallpaper and i'm looking for a dark theme.... 
Have you got any ideas ?

    Finally, i just want to thanks all the people who work for the XFCE 
project => you make a really good work !

    Thx U ++