This isn’t a new feature. Perch already allows you to add your CSS and JS to customise the control panel. So why the UI Loader app?
The default way to inject your own CSS and JS files to the control panel works, but Perch adds your CSS at the bottom of the page. While this is fine for JS scripts, it is not ideal for CSS particular if you’re making big changes as the browser will apply the default Perch styles first before your custom styles (so the user see the default styles for a second before the custom ones are applied).
The Perch documentation also mentions this:
(We appreciate that the end of the page is not the most super-ideal place to be adding new CSS files. If making big changes where this could be an issue, it may be betting to look at implementing your code with an app instead.)
The UI Loader is not an admin theme. The app comes with no CSS or JS. It just provides a way for you to specify what files you want to load, and it will load them for you.
For CSS files, the app adds them in the
<head> of control panel pages, so the control panel users will see your custom styles right away.
You can read more about the app and how to install it in the docs.
A number of developers have open-sourced their admin themes. They are listed in Perchology Links.