All you have to do to activate these themes is to enter about:config directly in the address bar, then search for Theme in the top bar as in the image, and you will see many commands and choices, then double click on browser.devedition.theme.enabled until the value changes to True to activate it so you can choose between the two appearances.
Now you will find that your appearance has changed to a wonderful one with bright and amazing colors, much better than the default appearance. If you are a lover of dark colors, specifically black, all you have to do is click on devtools.theme on the same page and write Dark instead of Light then OK. You will find that the appearance has changed to dark colors.
And now if you want to return again to the default appearance, all you have to do is repeat the first step with selecting False, where you double-click on browser.devedition.theme.enabled to change the value from true to False again, thus disabling the two appearances and easily returning to the default appearance.
Certainly, the first appearance, Light, is very distinctive for me as a lover of dazzling and bright colors, but the Dark appearance is also very distinctive for lovers of black and dark colors. This method does not consume RAM, does not cause Firefox to lag in any way, and does not affect the browser in any way, unlike installing add-ons and shapes completely.
