Previously, we used a version of libsass that did not permit importing CSS files inline. It only allowed 'CSS @import' or importing SCSS files inline. So, we used a build step to rename vendor "filename.css" files to "_.<filename>.scss", that would enable us to import them.
Now, thanks to
https://github.com/sass/libsass/pull/754 it is possible to directly import regular CSS files inline and so we should remove our now unnecessary build steps.