You can change the darkness of the foreground color by opening your add-on options. Navigate to about:addons, and click Options on the extension.

Could you please submit an issue with a more verbose description to the Github Issues page? I'm not sure what the problem is, and I would like to have major bugs fixed in version 2. https://github.com/spikespaz/firefox-nativedark/issues

This is not available for mobile because that is out of scope. The theme was made to adapt to your system color theme, and there is no way to do this on mobile, so you may as well just use a dark theme.

Unfortunately this can't be done without modifying userChrome.css.