How to style a custom widget?

I’ve created a custom widget and now I want to style it.
I know it’s possible to add styles to the css of dashboard/app but is there an option to have a css file specifically for the widget? Meaning, I want the file system structure to look like:

  • Project/
    • custom_widgets/
      • multiselect/
        • multiselect.py
        • multiselect.ts
        • styles.css
    • main.py

cc @mateusz what is the current best practice here?