fix single bg loading

autoload/zenbones/util.vim

@@ -5,13 +5,13 @@ function! zenbones#util#setup_colorscheme(...) abort
 
     let g:colors_name = a:1
     let l:bg = a:0 == 2 ? a:2 : v:null
-    if l:bg
-        set background=l:bg
+    if l:bg != v:null
+        let &background = l:bg
     endif
 
     if has('nvim') && (!exists('g:' . g:colors_name . '_compat') || g:{g:colors_name}_compat == 0)
         lua require("zenbones.util").apply_colorscheme()
     else
-        call {g:colors_name . l:bg ? '_' . l:bg : ''}#load()
+        call {g:colors_name . (l:bg ? '_' . l:bg : '')}#load()
     endif
 endfunction