neovim: set theme based on $TMUX env variable

neovim/.config/nvim/after/plugin/theme.lua

@@ -2,7 +2,13 @@ vim.api.nvim_set_hl(0, "SpellBad", {
   fg = "red",
 })
 
+local env = vim.fn.environ()
+if env["TMUX"] then
+  vim.cmd("colorscheme gruvbox-material")
+else
+  vim.cmd("colorscheme ayu")
+end
+
 vim.cmd([[
-colorscheme gruvbox-material
 hi clear SignColumn
 ]])