refactor(palette): Nightfox bg/sel conform to standard

[EdenEast]

This change brings nightfox inline with other palettes in regards to bg
and sel gradients. These colors where created from this code section:

local C = require("nightfox.lib.color")
local pal = require("nightfox.palette").load("nightfox")

local bg = C(pal.bg1)
local fg = C(pal.fg1)
local blue = C(pal.blue.base)

require("nightfox").setup({
  palettes = {
    nightfox = {
      bg2 = bg:brighten(6):to_css(),
      bg3 = bg:brighten(12):to_css(),
      bg4 = bg:brighten(24):to_css(),
      sel0 = bg:blend(C(pal.blue.base), 0.2):to_css(),
      sel1 = bg:blend(C(pal.blue.base), 0.4):to_css(),
      comment = bg:blend(fg, 0.5):blend(blue, 0.2):to_css(),
    },
  },
})

Old / New

Resolves: #155
Supersedes: #157