Feature Request: add markdown headings background

[luigir-it]

Hi.
I'm using Neovim 0.10 with markdown.nvim

Right now, headers foreground look really nice (I guess thanks to #196), but we can't say the same thing for the background
The highlight groups for the background H{1..6} link respectively to:
DiffAdd
DiffChange
DiffDelete (3 to 6)
And look like this:

I asked ChatGpt to make background colors that mix nicely with the foreground, and this is the result:

Much nicer!

The lua functions I used to make this happen (I'm using LazyVim distribution):

1. Define colors

{
    "sainnhe/gruvbox-material",
    init = function()
      vim.g.gruvbox_material_colors_override = {
        markbg1 = { "#2e1a1a", "52" }, -- Adjusted color code for #2e1a1a
        markbg2 = { "#332616", "94" }, -- Adjusted color code for #332616
        markbg3 = { "#3a3020", "100" }, -- Adjusted color code for #3a3020
        markbg4 = { "#243218", "22" }, -- Adjusted color code for #243218
        markbg5 = { "#1b2926", "23" }, -- Adjusted color code for #1b2926
        markbg6 = { "#2e1a20", "52" }, -- Adjusted color code for #2e1a20
      }
    end,
}

2. Define highlight groups

vim.api.nvim_create_autocmd("ColorScheme", {
  group = vim.api.nvim_create_augroup("custom_highlights_gruvboxmaterial", {}),
  pattern = "gruvbox-material",
  callback = function()
    local config = vim.fn["gruvbox_material#get_configuration"]()
    local palette = vim.fn["gruvbox_material#get_palette"](config.background, config.foreground, config.colors_override)
    local set_hl = vim.fn["gruvbox_material#highlight"]
    set_hl("RenderMarkdownH1Bg", palette.none, palette.markbg1)
    set_hl("RenderMarkdownH2Bg", palette.none, palette.markbg2)
    set_hl("RenderMarkdownH3Bg", palette.none, palette.markbg3)
    set_hl("RenderMarkdownH4Bg", palette.none, palette.markbg4)
    set_hl("RenderMarkdownH5Bg", palette.none, palette.markbg5)
    set_hl("RenderMarkdownH6Bg", palette.none, palette.markbg6)
  end,
})

[antoineco]

This looks nice indeed 👌

The use case of markdown headings is very specific, and this one applies to a specific plugin as well, so I guess it would be fine to use a blend of Gruvbox Material's foreground colors with the dimmed background color to obtain a pleasant effect.

The only thing that bothers me with making that change is that the colorscheme has multiple color variations. This would require quite a lot of off-palette colors to be included.