Make admin panel headers readable in Airy theme

The admin panel headers are white by default, making them almost
unreadable on the light background used in the Airy theme. Make the
headers dark, like text elsewhere in the theme.

mediagoblin/themes/airy/assets/css/airy.css

@@ -52,6 +52,10 @@ footer {
   border-top: 1px solid #E4E4E4;
 }
 
+table.admin_panel th {
+  color: #4a4a4a;
+}
+
 .button_action, .button_action_highlight, .button_form {
   color: #4a4a4a;
   background-color: #fff;