In .angular-cli.json, you can specify styles as input,output, lazy. If lazy, it will not include it in index.html (it is up to you). However, if --output-hashing is enabled (e.g. for production), it adds a hash value, so theme.scss becomes theme.6dc4d860143115033cae.bundle.css, not theme.bundle.css.
So, how do you know the actual filename? The only thing I can think of right now is, after the build find all .bundle. files, and create a json file that provides a map of the unhashed file name to the hashed, place that in assets, and load it as part of a service. But that seems a bit convoluted.