Hi Steve,
You can add an Expression type field in the .xml file, then you can add dynamic classes to that expression. You can then use that in the widget props to set the class on the rendered output of the widget.
<property key="dynamicClasses" type="expression" defaultValue="" required="true">
<caption>Dynamic classes</caption>
<description></description>
<returnType type="String" />
</property>
In the Render function:
return <div class={dynamicClasses}></div>;