This is the solution I found. 

### mytheme.libraries.yml

```yaml
mylibrary:
  version: 1.0
  css:
    theme:
      css/some/path/desktop-small.min.css: { media: "all and (min-width: 756px)" }
      css/some/path/mobile.min.css: { media: "all and (max-width: 755px)" }
```