Set background color for Normal group

[cubetastic33]

Set ctermbg for the Normal highlight group to 0. This is to fix an issue where some things were highlighting incorrectly when using vim-airline.

[upsetbit]

This merge introduced a new problem. When setting the background color group to 0, vim/neovim tells the terminal emulator (some of them) to overwrite transparency.

This is what I normally see:

This is what I see now:

I'm not sure how to fix this so for the time being, I'll just revert the commit locally.

I've tested on termite and alacritty on linux (manjaro). On MacOS (iTerm2) everything's still the same though.