Hi Josef, I’m able to set the border color and related properties from the area you’ve found.

Here’s a screenshot just to make sure we’re on the same page:

Is there a property you’re after that’s not here? If so, please try to clarify what else you’d like to change.

Hey Mark - I’m referring to that light blue/teal border in my screenshot. It surrounds the month and arrows. 

I see. That property doesn’t appear to have declarative controls in theme builder, but if you’d like to add some inline CSS to your theme (or to just one specific page), you can use a rule like this, and tweak the parameters that you want. 

.ui-datepicker-header.ui-widget-header{<br>&nbsp; &nbsp; border:2px solid red;<br>}