Header will no display correct on mobile phone

I would recommend to make a child theme so your changes in the style.css will be saved when there is a update.

In the style.css I would add the following code:

@media screen and (max-width:768px) {
	.site-header .site-branding { background-size: 100%!important;}
}

If you don’t want to make a child theme then you can go to:
https://arrowsandawe.com/wp-content/themes/sela/style.css
Rule 1762 and add the code above ??