Maybe it's because my eyes are getting old or maybe it's because the contrast between windows on macOS keeps getting worse. Either way, I built a tiny Mac app last night that draws a border around the active window. I named it "Alan".
In Alan's preferences, you can choose a preferred border width and colors for both light and dark mode.
If you want to hide Alan's icon from the Dock, you can set a hidden preference by running this Terminal command then relaunching the app.
defaults write studio.retina.Alan hideDock -bool true
That's it. That's the app.
You can download a notarized copy of Alan here.