A workaround ...
use the id
of the node in a selector (you find the nodes id in the sidebar under the info tab) e.g...
ui-template node (set for head)
<style>
[node-id="1f9886f1.f40569"] > button {
color: green !important;
background-color: purple !important;
}
</style>
but I agree, having the ability to specify an additional class
or classes on the UI items like button and label etc would simplify this somewhat.