When publishing a theme, Firefox says the theme is corrupt

I am trying to put together a theme for Firefox, and when I publish the theme and try to download the .zip or install it from the page, I get an error message that reads: “Installation aborted because the add-on appears to be corrupt.” The image size is 200x3000px, and 45.4kb.

The publishing settings I am using (if this helps):
Select a category for this add-on: Other
Can others share your Add-on, as long as you’re given credit?: Yes
Can others make commercial use of your Add-on?: No
Can others create derivative works from your Add-on?: Yes

This is the theme that I am trying to work on: https://addons.mozilla.org/en-US/firefox/addon/casino-night-zone-dark/