Question: I switched to a dark theme (Dolorean Dark). I like it and I think--it is better for eyes. However I have problems in some web pages. Most pages are intended to be shown on white background. On dark theme form controls have black background and mostly the font is also black, so the content of forms are invisible. I would like to use white theme on web pages in Firefox.

Answer: 
changing userContent.css solves problem ; more information is [here](http://ubuntuforums.org/showthread.php?t=1715679) also dolerean theme comes with the firefox tweak more information is [here](http://gnome-look.org/content/show.php/DeLorean-Dark?content=153866); just installing it is enough
To add on to Bannaz's [answer](https://askubuntu.com/a/778384/517280), use   

`env GTK_THEME=Adwaita:light firefox`  

if you want to launch via desktop/panel icon or keyboard shortcut.   
 This also means you don't have to modify `/usr/lib/firefox/firefox.sh`   
 Firefox 57 includes dark theme which can be enabled via Add-ons->Themes->Dark. [Screenshot](https://i.stack.imgur.com/9nc9I.png). This theme doesn't apply on webpage. Note: I'm using Ubuntu MATE 17.10

Alternatively, you can use this [add-on](https://addons.mozilla.org/en-US/firefox/addon/text-contrast-for-dark-themes/) (credit to [koushik](https://askubuntu.com/questions/765346/cant-read-text-in-web-forms-since-firefox-uses-dark-theme-by-default?noredirect=1&lq=1#comment1400374_772270)), or this [add-on](https://addons.mozilla.org/en-US/firefox/addon/re-style/) to apply the userContent.css (credit to [bkilinc](https://askubuntu.com/a/197014/517280)) in Firefox 57+.

I'm using Ubuntu MATE 17.10