While reduceTransparency is a legitimate workaround for the ugly grey menu bar, I find that it has too broad a system impact: effectively zapping transparency all over the place - especially the dock etc. You're left with a dock and notification sidebar panel (for example) that are barely visually functional. At least for my poor eyes...
I found a software switch in HIToolbox that simply turns menu bar transparency OFF. Unfortunately it is not available as a setting, environment variable or default - the equivalent of an HIMenuBarOpaque application plist setting would have been really nice, but Apple doesn't always make this easy.
Anyway, it's a one byte patch that easily (and harmlessly) flattens the menu bar to white or black (Light or Dark mode) but preserves all other functional transparencies. See below:
View attachment 781474 View attachment 781475
PM me if you're interested in the details or want to test the HIToolbox DB11 patch. At least it takes care of the grey menu bar (without affecting general transparency/vibrancy) which in my opinion is the most egregious aspect of the Light mode bug.
As far as AppKit and the ugly sidebars (in Finder and other apps), swapping Light mode processing for Dark mode is still an option (per my previous screenshots) but has some side effects I am not pleased with: dark window bars and obscured function bars in some apps. I've found that these "modes" are not consistently implemented - for example while my "Hybrid" mode looks good for the Finder AppStore and Xcode, News and iTunes look a little quirky. Still work in progress.