Don't hardcode colors

We use @theme_selected_bg_color for the blue 'stamps', so
it only makes sense to use @theme_selected_fg_color for
the text color.

src/gtk-style.css

@@ -29,7 +29,7 @@ GtkEventBox.gradient_box_software {
 	font-size: 80%;
 	background-color: @theme_selected_bg_color;
 	border-radius: 4px;
-	color: white; /* label color */
+	color: @theme_selected_fg_color;
 	text-shadow: 0 1px 0 rgba(0,0,0,0.5);
 }