Cannot Change Color in "Horizon" Theme

Hi! I've been working on a project that has this different approach from what we're used to doing. Because of this, I want it to look different, so I saw the Horizon theme and thought, this is wonderful! Definitely gives another vibe.

But the thing is that I encountered something that visually bothers me: This black bar on the cover page:

I already planned a color palette for this course and definitely that harsh black is not part of the look and feel. I tried to change it EVERYWHERE and no luck.

I can see it even in the Navigation section and there's no way to change it. 

It would be so cool to have a better and more complete color customization, 'cause honestly I wouldn't like to go to the code and change every single module that we produce with this theme. What do you guys think?