From MozillaZine Knowledge Base

userChrome.css in the chrome folder is a CSS file that can be used to change the way Mozilla applications' interfaces look. This file does not exist in a new profile. You can create it manually. If you just want to move/remove/rename menus and menu items or change/disable shortcuts use the Menu Wizard add-on instead.

Creating

To create this file, use your operating system tools.

Go to your profile folder. Then go to the chrome folder there, creating it if necessary. In the chrome folder, create a plain text file named userChrome.css. Copy and paste the following line as the first line of the file (if it doesn't already exist). This is called the namespace line and you must not alter it. Anything you add later on should always be below it

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

Notes: 

• If you have more than one profile, then each of your profiles has a separate userChrome.css file.
• There is another chrome folder where Thunderbird's code is installed. Do not create a userChrome.css file there—it will not work.

Editing

This file is designed to be manually edited using a text editor. Anything valid in a CSS file is valid in this file. To make the changes take effect, you must restart the application.

For Mini Tutorial (old) see Section 4 here -

[1]

For sample styles, see:

• userstyles.org
• Firefox Help: Tips & Tricks: Browser Appearance
• Category:Visual customizations (Firefox)
• Category:Visual_customizations (Thunderbird)
• Dialog too small or too large

Moving and copying

You can move or copy userChrome.css to a different profile. If it imports or references other files such as images, then you must also move or copy these other files. To make the changes take effect, you must restart the application.

Deleting

You can delete userChrome.css to get rid of any user styles defined there. To make the change take effect, you must restart the application.

Troubleshooting

For some information about common problems, see: Editing configuration – Troubleshooting

Related files and folders

• userChrome-example.css
• userContent.css
• userContent-example.css
• userChrome.js

See also

• Pane and menu fonts
• Chrome element names and IDs
• Editing configuration

External links

• The ChromEdit extension provides a convenient way of editing userChrome.css and other configuration files.
• The Stylish extension provides another way of changing styles in web pages and e-mails. It does not use the userChrome.css file.
• Customize your web stylishly provides a tutorial on using CSS and Stylish.
• The DOM Inspector, DOM Inspector Plus, InspectorWidget add-ons and the CSS Selector tweak in the obsolete MailTweak add-on are useful tools to identify the window elements.