move menubar

Likely but hard to provide a specific solution without a link or more info. You will likely want to edit the css using a child theme. If the elements are in a flex container, it could be as simple as specifying an order value.

https://backwardsd.com/
I am using the comicpress theme

Also what is a flex container? And if not, how would I need to edit it?

With the current setup, you could try this for the header:

div#menubar-wrapper {
    position: absolute;
    top: 0;
}

If your interested in flexbox, I like this resource, and this.

Did I not do it right? I put it in the custom css and it didn’t do anything.

Looks like it’s working to me. Maybe you need to refresh your browser or clear cache. Sometimes Chrome seems to not update css changes for me, so I like to test my changes in Firefox as well.

Ok it’s working but now it’s covering up part of the header image and is not spanning the whole page. What now?

In truth I can resize the header image if need be. I would like have the sidebars meet the top of the menubar situated between them across the whole page, forming a box around the header image.
Can I post an image here of what I mean if it would help?

Changing the layout of the page is going to require a bit of work. I think you would have an easier time if you first changed the markup of your page template by having the <div> for the menu (perhaps change to <nav>) placed above the <header>. Then I would approach it using flexbox.