The problem comes from the "Background Size" option that we have in our templates (see the attached screenshot).
If you set it to
Gantry 5 will show the warning when you recompile the CSS.
This is because of the new (more strict) SCSS compiler in Gantry 5.5
For now you have two options:
1. Just disregard the warning. Everything will work just fine for the foreseeable future.
2. Set the "Background Size" option to
We will fix this some day, but it is really not high in our priority list