Can I make the Keyboard Viewer more readable
In Catalina Keyboard Viewer, click the Gear icon in the upper right corner of the window. This opens a pop-up window where you can select the Appearance setting of the Keyboard View window to either Dark or Light, independent of the System Appearance setting:



When you choose Dark Mode, a pressed key is outlined in red.
When you choose Light Mode, a pressed key is outlined in red in addition to having a grey overlay, which is what you are looking for.