Quark how to change the header and footer?

Hi, Im not a programmer and Im trying to plough through the "code". How can I change the header and footer of quark? I found a footer pretty hidden in /user/themes/quark/templates/partials/footer.html.twig but where is the header? I`d like to have a background image or header banner.
There is no online editor of the theme? In my case it will just be a homepage of 5 pages or so.

@MarkusRTP, First of all, don't edit the templates of Quark itself. Create an inherited theme first. If not, you will loose all changes when a new release of Quark arrives.

Not sure if it fits your needs, but you could have a look at '/user/themes/quark/blog.html.twig' and '/user/themes/quark/modular.html.twig' which both override the empty {% block hero %}{% endblock %} of '/partials/base.html.twig'.

In this demo of the blog template, and this demo of the modular template, you can see the Hero image in action.

O.k., I`m using grav with quark with admin plugin online.

This introduction is very helpful and also explains the "hero" thing. I found that I don`t need to alter the template to get a background image installed. https://www.youtube.com/watch?v=3nGqaPl4i-M

Nevertheless I did this inheriting manually as I don`t have ssh at present and grav does not run on my linux pc for a reason.
Hm, how do I change the text color p ? Which css do I have to add where? Or is it a change in scss?
Here I read it would be in mytheme.yaml https://learn.getgrav.org/17/themes/theme-configuration so I added the line color: blue . Nothing.

Sorry, but in former times it was 1 style.css and I could do a few guesses.

O.k., I think I got it. I made a folder /themes/mytheme/css/custom.css and defined p there.