I finally got collective.multitheme released! https://pypi.org/project/collective.multitheme/ so @thiebovic you should be able to simplify your buildout.cfg changes down to just adding this one line:
eggs =
...
collective.multitheme
@thiebovic you need to use the command window, and issue a command like this:
C:/plonecms/buildout/env/Scripts/buildout.exe -c C:/plonecms/buildout/buildout.cfg
(thanks to @mauritsvanrees for zest.releaser, especially version 6.15.0 that fixes some new pypi.org-related issues)
Alternatively, you could move or copy your buildout.cfg so that it is in the same directory as buildout.exe (but that may cause other problems, though still worth trying)
Being it a theme, one or more screenshots could be very helpful 
P.S: https://gist.github.com/abhihebbar/8398586 this snippet can help to compile all the less file in subdirs.
Thanks you all for your help,
When i Run " C:/plonecms/buildout/env/Scripts/buildout.exe -c C:/plonecms/buildout/buildout.cfg" as @tkimnguyen told me, the command window tells me that:
"
Not upgrading because not running a local buildout command.
Unused options for buildout: 'eggs'.
_Updating mr.developer.
Updating instance.
Updating checkinterval.
Updating z3c.offlinepack.
"
I saw this post: http://plone.293351.n2.nabble.com/Buildout-Unused-Options-td3056454.html
Could it be useful? What is a recipe?
Thanks... I think I'm cursed..
For information: here is the WHOLE content of my buildout.cfg file:
"
#NOTE: This file should be copied into the buildout root directory. It
#should never be checked in to version control so that the environment
#stays the same over time.
#$ cd path/to/your-buildout
#$ cp profiles/buildout.cfg_tmpl buildout.cfg
[buildout]
index = https://pypi.python.org/simple/
#Once copied to the root, select the environment you are releasing to
#by uncommenting the appropriate line below.
extends =
#profiles/local.cfg
profiles/testing.cfg
#profiles/prod.cfg
#profiles/staging.cfg
eggs =
...
collective.multitheme
"
@thiebovic the ... is an ellipsis, meaning "anything that was there already", the point being that you add a single line to eggs, and that single line contains collective.multitheme
Yes, I realized it was a mistake but even without the "...", it doesn't work...
What about this part, is this normal? 
Buildout has run without errors and your instance starts successfully. What's the issue?
Hello @Rotonen !
My problem is that I added the multitheme add-on in my buildout.cfg and he doesn't appear in my add-ons list...
Here is my whole buildout.cf-g :
#NOTE: This file should be copied into the buildout root directory. It
#should never be checked in to version control so that the environment
#stays the same over time.
#
#$ cd path/to/your-buildout
#$ cp profiles/buildout.cfg_tmpl buildout.cfg
[buildout]
index = https://pypi.python.org/simple/
#Once copied to the root, select the environment you are releasing to
#by uncommenting the appropriate line below.
extends =
#profiles/local.cfg
profiles/testing.cfg
#profiles/prod.cfg
#profiles/staging.cfg
eggs =
collective.multitheme
"
@thiebovic a couple of notes:
</> to indicate preformatted text, otherwise any indentation gets eaten up by the editor. I will try to reformat your post above.You have to add it under the [instance] section 
I think this line is obsolete and should be removed
It says so in GitHub - collective/collective.multitheme: A multi purpose Plone 5 theme for Mosaic and Themingfragments
This is what I had to add so that the buildout.cfg that is in the repo would work for me last night.
... oops, just looked more closely: I added
index = https://pypi.org/simple
Here is the buildout.cfg that comes with the repo. It works. The add-on shows up in the Add-ons control panel. It may be overkill for you, @thiebovic, as it includes development tools, but as a baseline you'll know it works.
https://raw.githubusercontent.com/collective/collective.multitheme/master/buildout.cfg