Fix overrides not resetting fg or bg

[eeeXun]

The problem is when the group already has its own fg and bg, like DiffText. I want to override the bg color (see #314) but still retain the fg color, which is the syntax color provided by treesitter or LSP.

require("gruvbox").setup({
    overrides = {
        DiffText = {bg = "#6f5a2b"}
    }
})
vim.cmd("colorscheme gruvbox")

The
DiffText has the fg color bg0 and overrides doesn't clear fg color for me. So I can't get the syntax color provided by treesitter and LSP.

[ljden]

I don't think this is the right way to get the behaviour you're after - this clears the existing config, which means you are overriding more than just the bg

For example, the Todo group has the following config:

:highlight Todo
Todo           xxx cterm=bold,italic gui=bold,italic guifg=#282828 guibg=#fabd2f  

If I apply the following override:

overrides = {
  Todo = { bg = '#FFFFFF' },
}

I would expect the bg colour is set to white, and no other configs are changed.

The current behaviour is exactly this:

:highlight Todo                                                                                                                                                                                                                                                                                                                                                              
Todo           xxx cterm=bold,italic gui=bold,italic guifg=#282828 guibg=#ffffff   

Whereas this PR behaviour is:

:highlight Todo                                                                                                                                                                                                                                                                                                                                                             
Todo           xxx guibg=#ffffff