Use more discrete colors in dark backgrounds

[ayhon]

I recently started learning Coq, and when I installed this extension the first thing I noticed was that the CoqtailSent and CoqtailChecked's guibg values were too bright with my dark colorscheme. I fixed it locally by overwriting the highliights in my vimrc, but I figured this could be a change others would benefit from.

The colors themselves are just something I thought worked fine. I've also decided to leave the ctermbg values unchanged, since I couldn't find a way to open a vim session which used less than 16 colors, so I wasn't able to test different options.

[whonore]

Thanks for opening this PR. In general, I think people should just set the colors in their local configs since there's no way to pick ones that will work with all color schemes. However, in this case, I think it does make sense to have different defaults for light and dark, and the colors you picked seem to work pretty well with most of the dark schemes that ship with Vim.

As for the cterm colors and t_Co <= 16, I'm inclined to leave them alone until it becomes an issue for someone.