Take a look at this;

NoSquint {web link} NoSquint allows you to adjust the text-only and full-page (both text and images) zoom levels as well as color settings both globally (for all sites) and per site.

Thank you for your input, but unfortunately this extension affects the web page, I'm talking about the dev tools window from Firefox Developer (check the pic).

If anyone interested I posted this question also on Stackoverflow

You can look at this CSS file to see how the styling is done.

• chrome://browser/skin/devedition.css
• http://mxr.mozilla.org/mozilla-release/find?text=&string=devedition.css

This looks nice! Noob question tho: how can I access and modify this file? Where is it stored on my machine (win 7)?

You can't modify such a built-in CSS file because such files are stored in one of the omni.ja archives.

• resource:///chrome/browser/skin/classic/browser/
• resource:///chrome/browser/skin/classic/browser/devedition.css

You would have to write your own style rules and apply them via userChrome.css or Stylish. Use the !important flag to override existing style rules.

• https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity#The_!important_exception

The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration

Great! Thank's a lot for your time, if you wish you can answer the same thing on Stackoveerflow. I'll dig in this and if everything works fine I will share the monokai theme surely ;)