How to display a new liquid section template in the theme editor?

[ludi]

If I create a new liquid section template in the code editor how do I make that section template appear in the theme customizing editor, so I can add it to a page?

How can I edit the settings that appear in the theme editor for that new section?
E.g. I'm going to copy & then modify the image-banner. liquid section template in the Dawn theme, but I want to be able to upload 4 or more images, not 2.

When I create new . css files or any other asset files, how do I make sure they are added to the page?