[Nition]

Sorry, yeah, so you meant it'd be nice to have a feature/add-on that overrides the link colouring to be the same on every site. Absolutely. We had that once, in those distant days before CSS...

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Edit:

This works on Firefox:

- Install Stylus[0].

- Click the toolbar icon and choose Manage.

- Click "Write new style"

- Enter and save this:

    a:link {
        color: #0000EE;
    }

    a:visited {
        color: #551A8B;
    }

[0] https://addons.mozilla.org/en-US/firefox/addon/styl-us/

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Edit 2:

In Firefox WITHOUT installing any add-ons:

- Go to about:config

- Set toolkit.legacyUserProfileCustomizations.stylesheets to true

- Press Alt, click Help -> More Troubleshooting Information

- Next to "Profile Folder", click Open Folder

- Create a new folder there called "chrome"

- Make a file in that folder called userContent.css

- Enter the following CSS:

    a:link {
        color: #0000EE !important;
    }

    a:visited {
        color: #551A8B !important;
    }

- Restart Firefox

[Nition]

Turns out you can do what the top comment by chhickman is doing without any addon as well. Follow my steps above in "Edit 2", and add this to your CSS file:

    @-moz-document domain(news.ycombinator.com) {
      a:link {
        color: orange !important;
      }
      
      /* visited link */
      a:visited {
        color: green !important;
      }
    }

Per-domain custom CSS supported out of the box!