I have long wondered how to change my admin particularly for my custom modules. For many cases using an XML file is fine and I can use the built-in Joomla! form fields.
In some cases, I'd like to style them, which I can do by adding classes and then adding the content to the template file. Which is 99.9% of the time Isis.
But I have always wondered how I would achieve this on any admin template. I would need to load a CSS or JS file somehow. I could do this with an addScript declaration, but I do not have a PHP file to add this to. I could use an Admin Plugin, but that doesn't seem like the right solution.
And finally, how would I add an image, or some HTML e.g. an explanation or instructions. I can use the description panel on some occasions, but on others, it would make sense to be next to the field itself.