Change theme to gruvbox (#4)

This is the flavour of the month

lua/custom/plugins/theme.lua

@@ -0,0 +1,8 @@
+return {
+  'ellisonleao/gruvbox.nvim',
+  priority = 1000,
+  config = function()
+    vim.cmd.colorscheme 'gruvbox'
+    vim.o.background = 'dark'
+  end,
+}