I guess you are talking about the breadcrumb control in the skin file. You can change the separator there using any HTML code, but you have to HTML-encode the value. When you look at the "inner.ascx" file of the Xcillion skin, you find
Separator="<img src="/Portals/_default/Skins/Xcillion/Images/breadcrumb-arrow.png" alt="breadcrumb separator">"
as an example. Therefore, the solution should be
<dnn:BREADCRUMB ID="dnnBreadcrumb" runat="server" CssClass="breadcrumbLink" RootLevel="0" Separator="<span aria-hidden="true"> > </span>" HideWithNoBreadCrumb="true" />