Sass partial names must always start with an underscore

Be careful to name any file @imported by SASS with a leading underscore.

SASS files not beginning with an underscore will be rendered on their own, which will fail if they are using variables or mixins defined elsewhere. (For me it broke only in production, which may be due to some settings in SASS-GEM/lib/sass/plugin/rails.rb.)

From the SASS docs Show archive.org snapshot :

The underscore lets Sass know that the file is only a partial file and that it should not be generated into a CSS file.