create setup_colorscheme in vimscript

autoload/zenbones/util.vim

@@ -0,0 +1,17 @@
+function! zenbones#util#setup_colorscheme(...) abort
+    if exists('g:colors_name')
+        highlight clear
+    endif
+
+    let g:colors_name = a:1
+    let l:bg = a:0 == 2 ? a:2 : v:null
+    if l:bg
+        set 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()
+    endif
+endfunction

colors/zenbones.vim

@@ -1,11 +1 @@
-if exists('g:colors_name')
+call zenbones#util#setup_colorscheme('zenbones')
-    highlight clear
-endif
-
-let g:colors_name = 'zenbones'
-
-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 . '_' . &background}#load()
-endif