Change the antimatter header-image blue into another color

Where would I change the antimatter colorized header blue into another color, or set it to transparent?

I have found some clues in the file /user/themes/antimatter/templates/modular/showcase.html.twig - but changes in the file make no changes in rendering. Am I even in the right file?

Make shure to clear the cache! (https://learn.getgrav.org/admin-panel/dashboard)

Yes, cache is probably the problem here. While you're building the site, it makes sense to disable cache entirely. Find /user/config/system.yaml and make this change:

cache:
  enabled: false
---

Lol than, that makes sense. Still, some changes were showing up and some others weren't…?

Cache is disabled now. Changes to /user/themes/antimatter/templates/modular/showcase.html.twig still don't display.
I'm working with the antimatter blog skeleton. Am I missing something?

Don't forget to force a page reload in your browser or disabled it's cache or just close it and start it again. Aside: call me stupid or anything but personally I would love to see the whole concept of (desktop) browser caching to be history by 2017.