I hate the transparent menubar in modern macOS versions. I used to avoid this by enabling the "Reduce Transparency" option in System Preferences > Accessibility > Display.
In Mojave (on non-Retina systems) this has nasty side effects and generally makes everything look like garbage.
I found a hidden setting that apparently used to work, but has no effect in Mojave:
defaults write NSGlobalDomain AppleEnableMenuBarTransparency -bool false
My current poor-man's workaround was to create a 1920x1080 PNG with my preferred background picture, and add a 22px high 100% white bar at the top. This simulates the effect of having an opaque menu bar.
Does anyone know of a better way?
edit: to help illustrate the difference, check out the side-by-side comparison below. The image on the left is with my "hack" of placing a 22px high 100% white strip across the top of an otherwise solid gray background. On the right is the same transparent menu drawn on top of Apple's "stock" solid background. Note the reduction in readability/contrast: