You can just change the css extension to scss, import it and use mix.sass. I do it all the time with libraries such as animate. I change the name to animate.scss and import as I mentioned.
@ejdelmonico Thanks, that's probably the simplest way (didn't find any better solution). As far as I can see, if you are using SASS and if you need some "pure" CSS library (such as Animate.css) - there is no point in using npm, it's much better to download it to somewhere like resources/assets/css/libs (and change the css extension to scss...).
This is just a step by step version of @ejdelmonico 's answer.
Save the content of public/css/custome.css in resources/assets/sass/_custome.scss
You have to change the file extension to .scss and add an underscore at the beginning.
The underscore will be useful when importing in your main app.scss file.
Import your resources/assets/sass/_custome.scss file into your resources/assets/sass/app.scss this way :
/* importing _custome.scss from the same directory containing the following files
resources/assets/sass/{app.scss, _custome.scss, _variables.scss} */
// import _custome.scss
@import "custome";
// import _variables.scss (custom variables for bootstrap-sass)
@import "variables";
// importing bootstrap-sass from node modules (if you are using bootstrap)
@import "node_modules/bootstrap-sass/assets/stylesheets/bootstrap";