I believe this is currently a bug that the developers are looking to address (see issue #12290).

Hi Jeenu, if you want all of Firefox's built-in pages to have a dark theme, as well as websites that check your system theme preference, you could try this, or maybe you already are using this based on advice from the above link:

(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:

(4) Edit the value to 1 and then click the blue check mark button or press Enter/Return to save the change.

For reference, this value is used to answer web pages' inquiries about your preferred color scheme. Setting it manually overrides Firefox answering the inquiry based on your system theme:

• 0 => Light
• 1 => Dark
• 2 => No Preference

Success?

Is there a way to change the PDF reader to dark theme without altering the rest of the UI's theme? Also see my question on the topic or another relevant one.

Hi Václav, my answer in https://support.mozilla.org/questions/1305573 is the only way I know to modify the PDF viewer without affecting other built-in pages.