This was not so simple as Joomla starts a module box as MetaMod is published.... So you often comes to layout problems with an outer box for metamod modules, and then inner boxes for each indivdual modules.
The solution found is to give to metamod a false class suffix that don't exist in your CSS (nohilite or whatever you want as soon as it doesn't exist in your CSS).
Then the outer box of MetaMod is not styled, and you see either nothing, or the inner modules with their respective styles.
This module is simply a "must have" module in any Joomla site.