Revert "Bug 926385 - [Messages] saner button css rules r=julien"

This reverts commit 12fa8c5290f9073d4fcc530c398e446aa36bddf8.
Merge pull request #18292 from borjasalguero/revert_regression_mms

Revert "Bug 926385 - [Messages] saner button css rules r=julien"