Hey LeChevalierKANI,
I hope it is not too late, but I have found a solution to your problem.
Try using the Responsive CSS Editor plugin. You can use it set CSS for different screen sizes that you can set for yourself. You can also have multiple responsive CSS for different sizes.
You don’t need multiple CSS files for it.
Download link: https://www.wpwox.com/responsive-css-editor/
You can also find the tutorial on how to install and use in the link.
Cheers,
Karma
