added default bg-secondary which tracim_theme_builder will override

tracim/tracim/public/assets/css/bootstrap.css

@@ -6321,3 +6321,7 @@ button.close {
   }
 }
 /*# sourceMappingURL=bootstrap.css.map */
+
+.bg-secondary {
+  background-color: #F5F5F5;
+}