Thank you @MonicaCellio for the information about Stylus.
Because it seems most on-topic here, I will provide a list of attributes that can be used to modify link colors on different parts of the site using this extension.
To do this, you will need the Stylus extension provided here.
In the menu, add a new style and set the applies to field.

Now you need to add attributes for links. Each attribute looks like this:
.post-text a:link {
color: #6abb00 !important;
}
.post-text a:visited {
color: #bb0025 !important;
}
One is for the link before it is clicked, and one is for the link after clicking. If you know CSS, you probably already know how to do this.
The part of the attribute that changes with the link is the first part with the period before it. That changes depending on the "class" property of any div elements that hold the link. Here are the ones I used:
- post-text - links in the body of questions and answers.
- comment-copy - links in the body of a comment
- summary - links in the Top/Hot Questions list
- module - links in the Hot Questions/Related/Linked/Blog/FAQ sidebar
on the right of the page
- header - links in the headers of the top bar dropdowns
- modal-content - links in all of the top bar dropdowns
- nav-links - links in the left dropdown menu
- post-taglist - question tags
- comment-text - username of comment poster
- grid - links in the bottom of answers/questions, includes
share/edit/flag and username of poster and editors
- bottom-notice - links in the "not what you're looking for?"/"not the
answer you're looking for?" notice on the bottom of some pages
Just replace the text right after the period with the text you want. You can use multiple statements per line like this:
.post-text a:link, .comment-text a:link {
color: #6abb00 !important;
}
OR you can use multiple lines:
.post-text a:link {
color: #6abb00 !important;
}
.comment-text a:link {
color: #6abb00 !important;
}
Both of these examples will make unclicked links in posts and unclicked comment poster names colored.
If you are copying and pasting many lines of attributes, make sure the color is what you want before you start, because it will be a pain to change them all.
Here is the code I am using to change the color for all of the above attributes.

.post-text a:link {
color: #6abb00 !important;
}
.post-text a:visited {
color: #bb0025 !important;
}
.comment-copy a:link {
color: #6abb00 !important;
}
.comment-copy a:visited {
color: #bb0025 !important;
}
.summary a:link {
color: #6abb00 !important;
}
.summary a:visited {
color: #bb0025 !important;
}
.module a:link {
color: #6abb00 !important;
}
.module a:visited {
color: #bb0025 !important;
}
.modal-content a:link {
color: #6abb00 !important;
}
.modal-content a:visited {
color: #bb0025 !important;
}
.header a:link {
color: #6abb00 !important;
}
.header a:visited {
color: #bb0025 !important;
}
.nav-links a:link {
color: #6abb00 !important;
}
.nav-links a:visited {
color: #bb0025 !important;
}
.post-taglist a:link {
color: #6abb00 !important;
}
.post-taglist a:visited {
color: #bb0025 !important;
}
.comment-text a:link {
color: #6abb00 !important;
}
.comment-text a:visited {
color: #bb0025 !important;
}
.grid a:link {
color: #6abb00 !important;
}
.grid a:visited {
color: #bb0025 !important;
}
.bottom-notice a:link {
color: #6abb00 !important;
}
.bottom-notice a:visited {
color: #bb0025 !important;
}
If there are other links that don't appear colored, open Inspect Element and find a div that is a parent of the link. Use the class property in the attribute.