Add CSS class for the selected pocket piece

[VVestin]

fixes #12866
This change depends on this PR to lichess-org/chessground:
lichess-org/chessground#292

This is the styling I propose for the selected piece:

I'm definitely open to other ideas on how we should style the selected piece.
I copied that color from the selected piece background in chessground.
A similar style we use on the pocket is what we do for the pre-dropped piece where we highlight the background a shade of gray.

[kraktus]

Hey, thanks for the work, is this PR a draft due to work on your side pending, or because you're waiting for feedback?

[VVestin]

Hey, thanks for the work, is this PR a draft due to work on your side pending, or because you're waiting for feedback?

No, this is the only change I intend to make. It's a draft until the chessground PR gets merged: lichess-org/chessground#292.
(Also, I just realized the link I made in the original comment was broken! Fixed it now)

Is this the right way to handle linked changes? This was the advice I got on the lichess discord.

[TheForkPower]

@VVestin yes it is! Is this ready to merge?

[VVestin]

@VVestin yes it is! Is this ready to merge?

No, still waiting on the chessground PR to be merged:
lichess-org/chessground#292

Maybe I should reach out to @niklasf or @ornicar to take a look?