Improve CSS Aggregation & PET

I'm cleaning up a theme on one of my projects. Trying to improve CSS aggregation. I noticed pets is adding a global CSS file, but it's really only needed for a single form, which only pets admins can view.

Thus I've changed around the code a little to only include this CSS when this form is actually used.

A further improvement of this patch, would be to remove the custom CSS all together, and display the preview as a readonly textarea. Then is themes style this out of the box, it will look fine with pets. For my theme personally, the custom CSS is out of place and doesn't follow the same format as a standard field with a label would.

Either way, move forward with this patch, then decide if the second part I mentioned is worth implementing. For me, it doesn't really mater.