How can I change link colors in Firefox and Chrome, but leave all other colors and fonts as they are?

Some people who suffer protanomaly can hardly distinguish red text from black text on computer screens. Many web pages, like AskUbuntu.com, show visited and unvisited text links in red'ish colors. That reduces usability drastically for those affected.

How do I change the color of un/visited text links of Firefox and Chrome on Windows and Linux to classic blue'ish colors, thus making the web usable for people who suffer protanomaly?

The settings of Firefox and Chrome offer to override fonts and colors, for example, Firefox → menu Edit → Preferences → Content → Colors... → untick Allow pages to choose their own colors. But that changes all fonts and all colors of every element. That's not desired; just the color of text links, no more no less.

Answer

Firefox

You can do this by editing (or creating if it is not there) the file userContent.css. Its location depends on the platform (the parent folder can also be found in menu Help → "Troubleshooting Information" → section "Application Basics" → field "Profile Folder"):

Linux

• $HOME/.mozilla/firefox/[xyz].default-release/chrome/userContent.css

Windows

Depending on the Windows version and other configuration, userContent.css is in one of the following folders:

• 
  

  C:\Documents and Settings\[username]\Application Data\Mozilla\Firefox\Profiles\[xyz].default\chrome.

  
  


• 
  

  C:\Users\[username]\AppData\Roaming\Mozilla\Firefox\Profiles\[xyz].default\chrome (more generally, it is %APPDATA%\Mozilla\Firefox\Profiles\[xyz].default\chrome).

  
  

Common for Linux and Windows

You may have to create the chrome subdirectory and the file userContent.css.

Add these lines to that file to change the colors (obviously, you can change the actual colors used to whatever you prefer - e.g. using Paletton Live Colorizer):

a { color: #333388!important; }  /* Unvisited link color */

a:visited { color: #333340!important; }  /* Visited link color */

In most cases Firefox has to be restarted before it takes effect.

Also, for later version of Firefox (69 and later), check that toolkit.legacyUserProfileCustomizations.stylesheet in about:config is true. In some cases it is automatically set to true when file userContent.css is created, but that is not always the case.

Chrome

You can do the same for Chrome by editing $HOME/.config/google-chrome/Default/User\ StyleSheets/Custom.css which on Windows, is located at %USERPROFILE%\AppData\Local\Google\Chrome\User Data\Default\User StyleSheets\Custom.css.