The "Plain" theme from the Chrome Web Store:
appears to revert Chrome to its default settings.
I no longer have to squint to tell which is the active tab.
The icon for the "Tabs" extension is recognizable again, also without squinting.
This solves the problem that was why I came to this thread.
For a more generic solution that applies to different apps, here's a Terminal technique that resets the default for any specific app, even built-in apps: