Bug 375908 - Text in credits is invisible when default text color is white.

Set the text color to black.
p=Simon 'sipaq' Paquet <bugzilla@babylonsounds.com>
r=mkmelin

mail/base/content/credits.xhtml

@@ -55,6 +55,7 @@
     <style>
       html {
         background-color: white;
+        color: black;
       }
 
       * {