1)An option is to download and eventually purchase Boring Old Menubar from
https://www.publicspace.net/BoringOldMenuBar/
because it's shareware
2)Or take your favourite wallpaper - make it the same size as your resolution(or find a wallpaper that is that same resolution)
Open a photo or graphics editing software and open your wallpaper therein - then create either a vector layer or a pixel layer that spans the entire width of your wallpaper. Place this at the top where the menubar is and give it a color with a little more contrast in a green than you would think and make that I believe 24 px in height.
You can experiment with wallpaper sizes and see if they come out alright after editing. I only made my wallpaper with a red background behind the menubar for my exact monitor resolution. It worked out fine as seen in the November Desktop Screenshot thread on this forum or attached here
That last part was the free option if you already have a graphics editing program of sorts, Acorn, Pixelmator, Krita, GIMP, Inkscape, Photoshop, Illustrator, Sketch, Affinity Photo, Affinity Designer, Figma etc
Displaperture has been updated for Big Sur too. And that's free too. This software creates rounded corners like Boring Old Menubar does. It's the only reason I mention it along with my other comment. Displaperture has been around a lot longer than Boring Old Menubar.
Round display corners.
manytricks.com