make theme "simple" use white titles on dark backgrounds

This is similar to what is done in the "white" theme.

css/theme/simple.css

@@ -7,6 +7,9 @@
  */
 @import url(https://fonts.googleapis.com/css?family=News+Cycle:400,700);
 @import url(https://fonts.googleapis.com/css?family=Lato:400,700,400italic,700italic);
+section.has-dark-background, section.has-dark-background h1, section.has-dark-background h2, section.has-dark-background h3, section.has-dark-background h4, section.has-dark-background h5, section.has-dark-background h6 {
+  color: #fff; }
+
 /*********************************************
  * GLOBAL STYLES
  *********************************************/

css/theme/source/simple.scss

@@ -31,6 +31,11 @@ $linkColor: #00008B;
 $linkColorHover: lighten( $linkColor, 20% );
 $selectionBackgroundColor: rgba(0, 0, 0, 0.99);
 
+section.has-dark-background {
+	&, h1, h2, h3, h4, h5, h6 {
+		color: #fff;
+	}
+}
 
 
 // Theme template ------------------------------