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: 
The best solution to tell Firefox that it should use the light theme instead of the dark is by specifying the GTK\_THEME variable prior to launching.

So, 4 ways you get for doing this..

1- from terminal you can do:

```
GTK_THEME=Adwaita:light firefox

```

2- if you hate the terminal and wanna close it once firefox works:

```
GTK_THEME=Adwaita:light firefox &

```

3- from firefox about:config you can set the following setting to false:

```
browser.display.use_system_colors

```

**WON'T WORK FOR localhost SITES!** so if you are a web developer/designer it won't work out that well for you.

**4- if you want this to be the defualt behavior:**

Just add two lines to the Firefox launcher script located at /usr/lib/firefox/firefox.sh

**in the terminal type down:**

```
sudo gedit /usr/lib/firefox/firefox.sh

```

**look at the beginning of the script for this:**

```
MOZ_LIBDIR=/usr/lib/firefox
MOZ_APP_LAUNCHER=`which $0`
MOZ_APP_NAME=firefox

export MOZ_APP_LAUNCHER

```

**and change it to this:**

```
MOZ_LIBDIR=/usr/lib/firefox
MOZ_APP_LAUNCHER=`which $0`
MOZ_APP_NAME=firefox
GTK_THEME=Adwaita:light

export MOZ_APP_LAUNCHER
export GTK_THEME

```

**Finally, restart Firefox.**

Hope this helps.

Please help me by voting this up.

Thank you :)
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