Search and thou shalt find. All is to be found in different posts in this thread...
But to speed things up for you
- Change wallpaper: go to program/s2u2/gfx and replace 'wallpaper.jpg' and wallpaperL.jpg' respectively.
'wallpaper.jpg' should be portrait 240x320 and 'wallpaperL.jpg' landscape 320x240.
- Change clock font: same place, change files in folder 'clock'.
But first of all:
Make backups of the original files in case you want to go back.
Themes and clock fonts are all over this forum, but like I said, start by going through the posts in this thread.