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.lessfile 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
installthen place the
- Add the line below to your
[theme:genericsetup] install = install