Dark mode in Tor Browser

I want to enable full dark mode (all websites become dark) in the tor browser, not just a theme, as I am sensitive to white light. I can't use the tor browser for more than 2 hours (approx.) because of this white light. Is there any way to enable it without using any external add-on(s)?

What I'd tried?

I can see there is a setting, in about:config, named as "widget.content.allow-gtk-dark-theme."But when I enable it nothing happens.

Edit: I know it is more like a question, not an issue but, in my case, I can count it as an issue.