About Firefox Color

I have a xpi, zip and URL for my theme, I just have some questions:

Does the URL exist before I click ‘Share’?
Is the URL ephemeral?
If I drag the xpi into my browser Firefox says ‘This add-on could not be installed because it appears to be corrupt’. The console says ‘Add-on undefined is not correctly signed.
If I go to Themes then ‘Install Add-on From File’ the same thing happens.

Where are the color settings saved? Is it possible for me to manually amend them? I looked in about:config as well as prefs.js but couldn’t see any rgb or hex color settings that looked relevant.

Despite the theme being in use, Firefox says the enabled theme is ‘System theme - auto’ Is this expected?

Thank you.