#834 add new variable to dark theme. may need style review

Author: JoeGermuska

src/less/themes/dark/Variables.less

@@ -67,6 +67,8 @@
 @minor-ticks-line-color: 		darken(@color-dark,66);
 @major-ticks-line-color: 		lighten(@minor-ticks-line-color,10);
 
+@timegroup-message-color:       lighten(@ui-background-color,2);
+
 @brand-color: 					lighten(@color-background,25);
 
 @era-color-1: 					@color-theme-complement;