Allow user to override any entry in the default_theme

[mehalter]

This allows the user to override colors and highlight groups inside of init.lua

example:

local config = {
  default_theme = {
    colors = {
      bg = "#1e222a",
    },
    highlights = {
      TSField = { fg = "#C9CBFF", style = "italic" },
    },
  }
}

return config

Example using the color table:

return {
  default_theme = {
    colors = {
      bg = "#1e222a",
      lavender = "#C9CBFF", -- add a new color
    },
    highlights = function(highlights)
      local C = require "default_theme.colors"
      highlights.TSField = { fg = C.lavender, style = "italic" }

      return highlights
    end,
  },
}