osf how to preprocess css using rollup.js build

Summary:

in OSF how is css compiled and bundled for the browser. Also how can css preprocessing (like scss or sass) be added to an OSF project.

Content (please ensure you mask any confidential information):

In our OSF project we have found a use case for preprocessing css using a preprocessing library like sass or scss. Looking at the project it looks like all js/css build tasks are handled by rollup.js. Is there documentation or does anyone have experience adding css preprocessing? Also how is css currently compiled into a project.

Version (include the version you are using, if applicable):