fix default thumbnail path error

source/css/_variables.styl

@@ -34,7 +34,7 @@ logo-url = "images/logo.png"
 
 // Sidebar
 sidebar = hexo-config("sidebar")
-thumbnail-default-small = 'css/images/thumb-default-small.png'
+thumbnail-default-small = 'images/thumb-default-small.png'
 
 // Profile
 profile-avatar-size = 128px