header-image behind site-branding and outlining the branding

Ad the title suggests, I’m trying to put header-image behind site-branding (title and description), but not with a static image. I want to keep using the randomized uploaded header images. I can’t figure out how to get it done, should I even be messing around in header.php?

I’ve found that trying to re-order things so the div.header-image falls inside of the div.site-branding doesn’t work (image will just show up below branding), no matter where I place it in the code. I can’t figure out what to do with the style.css file either, adding some padding doesn’t seem to work.

I don’t know how to code, but I can’t make any of the suggestions I’ve found online work.
This thread suggested the padding I couldn’t get to work for me and this thread uses a static image that I want to be randomized as per the theme.

This seems like something I should be able to figure out but after 10 hours or trial and error I’m at my wits end.

The relevant website is joris.victoryatsea.nl

Oh! And, since the images aren’t always nicely color coordinated I’d like to “outline” the site-branding too. Google has led me to believe that text-shadow is the way to go, but I don’t know how to implement it to the text. Is that something that should end up in styles.css?

Thanks for reading!