The Multiselect control hardcodes the style of the Placeholder to Arial, 13px. Please make this either have HtmlAttributes or put a style on it, that can be overwritten.
This is achievable by specifying the k-multiselect-wrap styles with higher specificity, as explained in this forum thread: http://www.telerik.com/forums/multiselect---data-placeholder---style To my understanding this is what you are after, let me know if not.