Go to your AppVM, and search for a file named something like ~/.librewolf/<something>.default-default/prefs.js
That is the pre-config of about:config of your browser instance. Add this lines at the end:
user_pref("ui.systemUsesDarkTheme", 1);
user_pref("layout.css.prefers-color-scheme.content-override", 2);
user_pref("browser.theme.dark-private-windows", true);
user_pref("devtools.theme", "dark");
user_pref("privacy.resistFingerprinting", false);
user_pref("privacy.fingerprintingProtection", true);
user_pref("privacy.fingerprintingProtection.overrides", "+AllTargets,-CSSPrefersColorScheme");
Last three lines (privacy tweaks) are needed to add because, if not, librewolf keeps using light mode to enhance privacy.
Privacy tweaks reference here: https://www.reddit.com/r/LibreWolf/comments/1d9d3k4/dark_mode_websites_despite_resist_fingerprinting/
I am not losing that much privacy for using a dark mode, imho.
I would like to apply this at TemplateVM level, but I haven’t been able yet, because TemplateVMs will have different folders under .librewolf than its AppVMs. So yeah, you must do this manually in each AppVM…
Edit:
For disposable VMs, you can add it to the disposable template.
For example, let’s say you have the default default-dvm. Open that template, not the disp???? one. There, browse to the ~/.librewolf/<something>.default-default/prefs.js path. Apply the patch and shutdown the Disposable template. Now any disp???? you open will inherit the dark mode patch.