Is there a way to customize the color of visited links? I am an Apple user, and am currently using the Chrome and Vivaldi browsers. Not a computer programmer. Is there any easy way to make visited links stand out? The default color for those isn't defining enough for me, I would like the color to be more noticeable.
There is. 

The color of a visited link is determined by the people who have designed the website you are visiting. Most sites will show links and visited links in the same color, but Google for example use different colours so that you can see which sites you have visited. This can be useful if you're trying to find back the site you've visited, or rather if you don't want to visit this site again and want to try another of the search results. 

The way Google does this is by putting this into something called a Cascading Style Sheet (CSS), which are effectively instructions for your browser on how Google wants it to look in the browser. CSS allows for different link 'states' to have a different color.

Some examples ... 

/* unvisited link */
a:link {
    color: green;
}

/* visited link */
a:visited {
    color: green;
}

/* mouse over link */
a:hover {
    color: red;
}

/* selected link */
a:active {
    color: yellow;
}


In addition to this, some browsers allow you to "overrule" the way a site operator wants their site to look by letting you apply your own CSS to a site.

Chrome used to allow users to directly enter this type of custom CSS. Nowadays, Chrome only allows this through the use of a plugin. Stylebot is one of them, Stylish another. 

Vivaldi is a browser I'm not familiar with. A quick Google search indicates that potentially aforementioned Stylebot can also be used for Vivaldi, but perhaps someone with more expertise on this specific browser can help out on more specific instructions.