It's easy to change the area around the document, but Firefox doesn't have a built-in feature to modify the appearance of the document itself. For that you would need to delve into the world of userContent.css files, which is tricky for reasons I'll save for later.

Dark Theme Preference

Firefox generally tries to follow your system-level preference for a "dark theme" on most internal pages like the PDF viewer. If your system isn't setting that flag, you can create a new preference to tell Firefox (and websites that care) about your dark theme preference. Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box, type or paste ui.systemUsesDarkTheme

(3) Firefox should display a bar with some radio buttons to choose a new preference type, followed by a + button. Choose Number and click the + button:

(D) Edit the value from 0 (Light) to 1 (Dark) and then click the blue check mark button or press Enter/Return to save the change.

Success?

If you want to return Firefox to figuring this out for itself, click the trash can at the right end of the row to remove this preference.

These posts discuss how to change the colors in the PDF itself:

(1) Invert the colors using a simple style rule in userContent.css

This lacks flexibility because it's global.

https://support.mozilla.org/en-US/questions/1281116#answer-1295331

(2) Create a "bookmarklet" to modify the colors

A bookmarklet is a bookmark that has a script as its address instead of a regular URL. A click on the bookmark runs the script in the context of the current page. So while this is not automatic, it gives you more control.

https://support.mozilla.org/en-US/questions/1281116#answer-1295338