I've used the ui_template node to add to and/or modify the base dashboard styling, as well as for including other javascript libraries. It does mostly work, as long as your images/stylesheets/libraries are local to node-red, as Cor mentioned. But because those resources are loaded asynchronously, they may or may not show up properly in the rendered dashboard pages.
I have also noticed that the group still takes up some room on the page, even if the title is turned off and there are no visible html elements in it. Is there an option to make a group not displayed? A while back we discussed the possibility of either "Page" level and/or "Dashboard" level groups that would be rendered in the dashboard pages before the visible elements, but take up no room -- Dave, is that something that still makes sense, or does the new styling feature make this unnecessary?