Feature Proposal - Different highlights for all three windows

[weilbith]

Hey 👋🏾

I have a local feature that allows me to set different highlights for the different windows. I basically use it to have a more decent/tidy look by using a different background shade for the middle window in contrast to the outer windows.

To make this a proper feature, I would add the 6 new highlight groups (3x background + border) and link them per default to the "regular" 2 highlight groups. Thereby it would be also backwards compatible.

What do you think about this? Would be fine with me opening a PR for this? Or do you have some early feedback about the approach?
Thank you! 🙏🏾

[SmiteshP]

As long as its backward compatible its fine ig