From left to right let ayucolor = light, mirage, dark

With let g:ayu_extended_palette = 1:

ayu only works if VIM supports termguicolors option. This is true for Neovim and VIM from 7.4.1799.

Plug 'Luxed/ayu-vim'    " or other package manager
"...
set termguicolors       " enable true colors support

set background=light    " for light version of theme
set background=dark     " for either mirage or dark version.
" NOTE: `background` controls `g:ayucolor`, but `g:ayucolor` doesn't control `background`

let g:ayucolor="mirage" " for mirage version of theme
let g:ayucolor="dark"   " for dark version of theme
" NOTE: g:ayucolor will default to 'dark' when not set. 

colorscheme ayu

Here is a list of plugins which have been customized to work better with this theme:

• vim-gitgutter. Git sign column signs.
• vim-signify. General VCS sign column signs.
• NERDTree. File explorer.
• telescope.nvim. Fuzzy finder (neovim only).
• YATS. Yet Another TypeScript Syntax.
• fugitive.vim. Git helper.
• startify. Start screen plugin.
• vim-pandoc-syntax. Pandoc specific markdown syntax.
• haskell-vim. Haskell syntax highlighting.
• nvim-compe. Neovim completion plugin.
• nvim-ts-rainbow
• nvim-cmp. Neovim completion plugin, successor to compe.
• indent-blankline.nvim. Indentation guides.
• hlargs.nvim. Highlight arguments' definitions and usages.
• vim-illuminate. Automatically highlight other uses of the word under the cursor.
• nvim-lspconfig. Neovim lsp config quickstart.
• nvim-navic. Simple winbar/statusline plugin that shows your current code context.
• coc.nvim
• modicator.nvim. Cursor line mode indicator.
• vim-floaterm. Terminal in floating window.

And here is a list of other supported syntax groups:

• Vim8 and Neovim Terminals.
• Diff syntax.
• Netrw.
• Neovim's Built-in LSP.
• TreeSitter (Neovim only).
• Javascript.
• Vimscript.
• XML.
• INI.
• Shell.
• PHP.
• Ruby.

let g:ayu_italic_comment = 1 " defaults to 0.
let g:ayu_sign_contrast = 1 " defaults to 0. If set to 1, SignColumn and FoldColumn will have a higher contrast instead of using the Normal background
let g:ayu_extended_palette = 1 " defaults to 0. If set to 1, enables extended palette. Adds more colors to some highlights (function keyword, loops, conditionals, imports)

In your Tree-Sitter configuration add the following (in lua):

require('nvim-treesitter.configs').setup{
  rainbow = {
    enable = true,
    colors = require('ayu').rainbow_colors()
  }
}

Currently, the ayu-vim theme supports the following statusline plugins:

• lualine.nvim. No setup is required. The theme is automatically applied.
• vim-airline. No setup is required, but it is important to note that the ayu theme included in vim-airline-themes will override the one present here.
• lightline.vim. Add the following to your config (or modify your current lightline config): let g:lightline = { 'colorscheme': 'ayu' }

This is an example for lualine.nvim:

Light:

Mirage:

Dark:

function! s:custom_ayu_colors()
  " Put whatever highlights you want here.
  " The ayu#hi function is defined as followed:
  " ayu#hi(highlight_group, foreground, background, [gui options])
  " See autoload/ayu.vim for color palette. 
  " `foreground` and `background` are required while the gui options are optional
  " `gui options` only represents the values you could put in the `gui` part of the highlight. See `:h highlight-gui`.
  call ayu#hi('IncSearch', '', 'vcs_modified')
endfunction

augroup custom_colors
  autocmd!
  autocmd ColorScheme ayu call s:custom_ayu_colors()
augroup END

Doing this will allow you to modify the theme to your liking while not modifying the colors for any other theme.

• ayu-vim: The official ayu-vim theme. It is currently un-maintained.
• neovim-ayu: A reimplementation of the theme in lua for better Neovim and lua support.