In the most recent update I was disappointed to see the color changes made. 1. The assumption that removing a choice for the white color of the browsing window was in my opinion not a good idea. I say choice because I already have dark reader which allowed me to use a dark field or light field as and when I wanted. I have no choice now. 2. The color green of the top icon site bar is not well thought out. Each person has a color preference and I am sure that there are people that like green and don't like green. It would be the same thing for any color scheme that was selected. So the real issue for a browser designer is functionality. How well does the color and or tonal contrast aid the viewer in seeing the printing clearly. In the new color update the color contrast has been lowered due to the tonal value of the green thereby affecting the contrast and clarity of the printing. I think decorative changes as opposed to functional changes should be considered thoroughly. There is less chance of causing unwanted results by sticking with functionality first and decoration better left with options so that a individual user can select what they would chose to see rather then assuming that the designer taste is what is appropriate for everyone. In short I believe you had the best functionality in my previous version.
One of the built-in themes is named "Light" -- that should give you a light theme for sure. You also could review the "Website Appearance" section of the Settings page (on Mac, it might be called the Preferences page). This article shows the details: https://support.mozilla.org/kb/change-web-appearance-settings-firefox (below image is from the article).