Is there a way to create a “style sheet” for Unity 5’s UI system? I want to be able to create all my UI, and then change it visually as well as the fonts in one place, thus automatically changing it everywhere.
No, there is no built-in way to do this. I think the easiest way would be to create some simple images that you expect to need, like “panel_9slice”, “button_9slice”, “icon_background”, etc. Then when you’re done you can just overwrite the files and they will update everywhere, as they’re still being referenced at the object level.
Same for fonts, though you might want to change the names from something like “Arial” to “score_font” and the like, just to prevent confusion when you’re overwriting it and suddenly have a Helvetica called Arial (though you can still rename the asset without worries; just make sure to do it one step at a time (i.e.: overwrite → rename or rename → overwrite)).
If you want more programmatic control, you would need to roll your own solution.
You might be interested on the Coherent UI.
You can get some of this with prefabs. But its not entirely the same functionality as a style sheet
Thanks everyone