[Theme:Hemmingway Rewritten] Make black box transparent und font-color black

I’ve tried each and everything and am close to giving up now.
I’m using the theme Hemingway rewritten and was already able to scale down the header image. Now I want to make the black box in the header transparent and change the color of the blog title to black.

Here’s what I have right now: https://schreiblust-leselust.de/

I’ve already tried adding the following to custom-header.php. Maybe I’m putting it in the wrong place? Since neither of it worked …

1)
.site-branding {
background: rgba(29, 29, 29, 0.84);
}

2)
.site-branding {
opacity: 0.8;
}

3)
.site-branding {
background: transparent;
}

Hope to find help here -.-
Next step I want to take then is changing the font color of the black box’ blog title into black – how would you do that?