I use rails and bootstrap by using this bootstrap-sass
When in Rails, there is a special import syntax that allows you to glob imports relative to the folder of the stylesheet that is doing the importing.
@import "mixins/*" will import all the files in the mixins folder
@import "mixins/**/*" will import all the files in the mixins tree
Scss being a superset of css, you can just change your .css extension to .scss, it will still work in the same way.
Then you can require them like this, and it should work: