Make floating window padding same color as inner rectangle

[Nargonath]

When using borders on floating windows, we have a padding between the window content and the actual border. This padding is not using the same colour.

[Nargonath]

The highlight groups to use are probably the one listed here: https://neovim.discourse.group/t/how-to-configure-floating-window-colors-highlighting-in-0-8/3193