See crash_springfield's comment at https://www.drupal.org/node/2615568/discuss#comment-12891366 →
We need to figure out why Option #4 from https://www.drupal.org/docs/8/modules/sitemap/overriding-sitemapthemecss... → no longer works, provide the correct solution, and if possible reference when the change occurred.
Closed: outdated
1.0
Documentation
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.
Going to close this (as recommended in #2), with a link to the documentation page for the process that @smustgrave recommended, Adding assets (CSS, JS) to a Drupal theme via *.libraries.yml: Overriding and extending libraries →