Actually, you should not be adding your custom sass rules to indigo/theme/lms/static/sass/multicourse/_course_about.scss – my assumption above was incorrect. In fact, you should simply add your rules to indigo/theme/lms/static/sass/partials/lms/theme/_extras.scss. I did just that by adding the following rule:
.course-info header.course-profile .intro-inner-wrapper .intro > .heading-group {
border-bottom: 15px solid red;
}
Which is successfully applied:
If that works for you I think you owe me a beer 