This assumes that you're using collective.themesitesetup. I don't recommend that you use this fullwidth implementation on pages where side portlets are enabled, it tends to look "wonky".
- Include the
fullwidth-for-mosaic.less
file with your theme, it needs to be included as part of one of the files that gets compiled to css. - Create a folder in your theme and name it
install
then place theregistry.xml
in theinstall
folder. - Add the line below to your
manifest.cfg
file
[theme:genericsetup]
install = install