Sure, multiple style sheets can be applied. Whether or not we generate some CSS behind the scenes, the user should always remain able to specify their own style sheets.
The style sheet options take a space separated list of URIs, absolute or relative to the input. But for simplicity I think it will be fine to limit it to a single absolute file URI, so that we can indeed use a file picker.
Originally posted by @bertfrees in https://github.com/daisy/pipeline-ui/issues/191#issuecomment-1942291915