vim: set colorscheme background to NONE

https://github.com/vim/vim/issues/10449

vim/vimrc

@@ -72,6 +72,9 @@ syntax enable
 set t_Co=256
 
 set background=dark
+augroup transparent_bg | au!
+  au Colorscheme * hi Normal ctermbg=NONE
+augroup END
 colorscheme desert
 
 set number