+= document background option 'white' properly

doc/neotheme.tex

@@ -332,9 +332,9 @@ The list of options is structured as shown in the following example.
   and |example|.
 }
 
-\DescribeOption{background}{dark, light}{light}{
+\DescribeOption{background}{dark, light, white}{light}{
   Provides the option to have a dark background and light foreground instead
-  of the reverse.
+  of the reverse. White is similar to light but uses a pure white background.
 }