I am using icons for my Squarespace Courses sidebar lesson thumbnails and although my PNG file is transparent the thumbnail is not transparent on the sidebar.  I have figured out how to remove the play button (thank you Becca Harpain) and update the background color on lessons that are NOT active

.course-item__side-nav-thumbnail {
   background: #F1F1F4;
}

How would I modify this to update the background color on the ACTIVE lesson?

Is there a way to share a link? Or is it in a private page?

@sorca_marian I don't want to share my in process course but I mocked up a little test course that uses the same icons and color themes and uses the CSS above to make them gray on the side navigation bar.

https://shannahalbert.squarespace.com/test-course-fgKgG 

@sorca_marian  Yes!  That worked!  Thank you so much.  (I had to remove the CSS I had in there that was setting the background when the thumbnail was not active - which was clumsy CSS guesswork!) Your solution is much more elegant and works when its active and not active which is clearly the way to go.  Thanks again!