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

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 Directory" ("Profile Folder" on Windows)):

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 versions 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.