Use my colors
From MozillaZine Knowledge Base
This page addresses web page colors with some conceptual alternatives to use of Tools, Options, Content, Fonts and Colors to specify font, links, and background colors and whether to allow pages to override your default color choices.
Remove all styling with 'No Styles' styling
A quick way to view a page with bad color choices is to remove all styling, which you can do with View, page styles, no styles, and return to normal with Basic style. That includes formatting as well as colors, and is a quick way of making a page readable.
Remove all styling with an extension
A quicker way to get an equivalent of 'No Styles' is the Read Easily extension which can be toggled on/off with a toolbar button, context menu item or a hotkey (CTRL+SHIFT+Z) to disable/enable styles on web pages. Applies to the current web-page. (Although nothing changed in Firefox to make the extension actually incompatible, you will currently have to override non-compatibility to use this small extension in Firefox 3.)
An example would be zap colors bookmarklet on the Bookmarklets for Zapping Annoyances page at squarefree.com. Click on the "zap colors" button on that page to see change. The button is actually a link. You can drag the link to a folder on your toolbars, and give it a keyword shortcut so you can implement it from the location bar, or drag it directly to the bookmarks toolbar itself and use it like a button.
use of userChrome.css
You can make global styling changes, with userChrome.css and userContent.css but it takes restarting of Firefox to implement and it is not very practical for most users for this type of a change as they would want to change things or instantly turn changes on or off.
use of Stylish extension
According to this bug report "The 'Stylish' extension was recently sold and the new company now logs all browser history.".  Supposedly Stylus is a fork of that extension without the spyware. See https://forum.userstyles.org/categories/stylish for a discussion about what's going on. Another fork is Osprey but they have no plans to support Firefox, recommending you use the more bare bones stylrr add-on instead.
Styles from userstyles.org are used with the "Stylish" extension and would be the same as having them in your userChrome.css to change your chrome appearance. But stylish allows you to have separate style changes that you can turn on/off and use them immediately without restarting Firefox. You can read about Stylish at userstyles.org and can install Stylish at addons. Stylish resides on your computer and is under your control. Move the Stylish button to your toolbars using Customize.
You can install a style into Stylish as a global style to change all web pages, as in Zap Colors style, which is similar to the Zap Colors bookmarklet. This particular example would not be as practical as a toolbar button, or a bookmarklet because it overrides all web pages and you have to go through Styles Manager to change. But you could easily convert it to be site specific by creating a new style modified to be site specific. Another style of interest might be Inputs Default to Black on White style.
But there are a lot of Global styles available at userstyles.org that actually are intended for global use, and many are comparable to Themes used in Firefox.
On a per site basis, you can use a style from userstyles.org that is specific to a site(s) or domain(s). Examples can be found in Site Styles at userstyles.com.
Most of the styles at userstyles.org are probably site specific styling changes, each site, or groups of sites would be another style, so there is a big proliferation in that area of styling changes way beyond simple color changes.
Some of the extensions involved with colors may be very large like 100kb so you might want to check carefully that it has features that you actually want/need.
Extensions to help change things but don't change the colors in themselves.
Changes to the chrome is a huge item with Firefox and Mozilla browsers that there is a whole category named Themes that are installed as Themes and not as an Extension. You can have a look at the Themes available at addons.mozilla.org
Test Colors Changes
Test Color changes (CSS):, , , , , , , , , , , , , , ,