I did a compton modification to add custom colored shadow for the focused window a while ago. I didnt submited a pr back then because it was experimental and that compton seemed unmaintained.
Now i found this fork and I would love to do the switch but I really prefer having borderless and colored shadow to show me which one is selected.

Here is the options I added in https://github.com/adberard/compton

• add the use of hex colors (in compton.conf: shadow-hex "#XXXXXX", in option : --shadow-hex 0xXXXXXX) for shadows (--shadow-hex)
• add an option to customise the focused window shadow color (--shadow-focused-hex)