can you share how you try to use it ?
xaringan won't be able to find itself your css file. you need to provide the whole path to your css.
I think having a custom theme in it own package, you would need to create a custom format in your package that is calling xaringan from your package and linking to the css file.
Otherwise, your package can offer a function to copy a css file in local folder, or offer a template that would create a folder with all assets.
If you share more on how you are currently working, I can try to give examples.