The LearnDash course list component includes an enrollment status "ribbon" displayed with each course in the list. Here's how to style the ribbon background color and text.
Style the LearnDash Course List Ribbon with Divi LearnDash Kit
The Course List module added by Divi LearnDash Kit includes built-in options for styling the status ribbon. You'll find these settings at:
Course List Settings > Design > Ribbon
These options are available in Divi LearnDash Kit v1.6.0 upwards.