Prior checking user defined theme

plugin/airline.vim

@@ -76,7 +76,7 @@ function! s:init()
     call airline#extensions#load()
 
     let s:airline_theme_defined = exists('g:airline_theme')
-    if !airline#switch_matching_theme()
+    if s:airline_theme_defined || !airline#switch_matching_theme()
       let g:airline_theme = get(g:, 'airline_theme', 'dark')
       call airline#switch_theme(g:airline_theme)
     endif