Header repeats horizontally and vertically

Try this:

header.home-header {
    background-repeat: no-repeat;
    background-size: cover;
}

To add or override CSS: use the “Additional CSS” option in the customizer. https://codex.wordpress.org/CSS#Custom_CSS_in_WordPress

Learn to use the Chrome Developer Tools or Firefox Developer Tools to help you see and test changes to your CSS.

See https://i.imgur.com/G7TxyFe.png

Alright, the change worked when I used Chrome Developer Tools, but it’s added in Additional CSS, there’s no change, I’ve published it and nothing. This may be a dumb question, but am I not saving the CSS properly? Thanks in advance!

I don’t see that CSS on the page. Do you have any caching plugins?

I don’t believe I do, no. Should I?

Caching will speed up your site, but in this context, it might obscure changes to the site.

Please show a screen shot of your Additional CSS window.