Hi
you can fix color issue with following CSS code:
.jsndatepicker.dropdown-menu {background:white}
Is there a CSS for the datepicker that would contain ALL of the datepicker items so it will display properly no matter what template CSS uses?
Sorry but generally this is not impossible, anyway, we use Bootstrap classes to try to fit the most of Joomla templates and this way normally works, but in your template there is a strange CSS code:
Bootstrap uses a class called "dropdown-menu" to assign style for dropdown windows (
http://getbootstrap.com/components/#dropdowns), but, your template for this class assign background-color (#444) and do not assign a text color (should be white), see screenshot please.
So in this case this seems only a template issue.