Ufc color codes not showing in builder responsive mode.

In responsive mode I select "mobile (315px)".

I add this custom rule:

.upfront-region-container-header{
 border-top: 1px solid #c5c5c5;
 border-bottom: 1px solid #c5c5c5;
}

This works fine and it instantly shows the border in the editor window.

If I try this rule instead:

.upfront-region-container-header{
 border-top: 1px solid #ufc1;
 border-bottom: 1px solid #ufc1;
}

Then is doesn't show the CSS in the builder. If I save it and view the site then the CSS works fine and the color code is replaced but even after reloading the builder this change is not visible.

To re-iterate. The custom css rule on the breakpoint works for the front end site but it is never shown in the editor when using a #ufc color code.

In other parts of the app it's fine to use the color codes so I think there may be an issue here and I would prefer to be able to see the CSS styles in the builder.