My french would be much worse than your english, but let me try to see if I understood. You have two issues (I think)
1. How to get a "theme" to work in a win7PE? (not sure anyone has done this, but do some searches here). In general, a theme is a complex set of settings on sounds, wallpapers, screen savers, etc. Maybe a simple wallpaper would be enough? And I know that there are scripts to help do that.
2. On "Multi-boot"...there are also some good scripts that manage the basics using either syslinux or grub4dos in a limited way, but adding in the option to boot to the PE or a linux should be possible. In win7PE_SE, there are scripts to manage "OtherOS" types, and I'm pretty sure you may find something there. Peter's MultiPE also has some capabilities to help manage the bootloaders, and there are a lot of other bootloader (grub4dos and/or syslinux) threads here to search through (and to use google translate on
I have also been working on a winbuilder based "multiBoot" project for personal enjoyment, but haven't had the time (yet) to polish it up for public exposure...
Good Luck, and WELCOME