Hmmm, sorry about thavt. this seems to a bag with the edit option in DW CC.
a work around would be to create a new blank page just as a testing page. apply the date picker to that page with the theme you wish to use.
locate the CSS line on that page, something like:
<link type="text/css" href="webassist/forms/fd_swisswebfactory_bravehearts/Datepicker/css/jquery-custom.css" rel="stylesheet">
and replace it on the other page.
I Have logged this bug in pour system to be corrected in the next update.