Problem customizing CSS in Gravstrap

Hi, newbie question here: I have been playing around with CSS in the learn2 theme without any problems, but I am having trouble with the cached CSS in the gravstrap theme. From what I can tell in the base.html.twig file, first bootstrap, then gravstrap and finally custom.css is processed. I have added a custom.css to the the css folder, but this file does not seem to be included in the cached css-file in the assets folder despite clearing the cache and reloading the pages. What am I missing here? What's the best approach to debug this? Thanks so much!