Most of those who are on shared hosts doesn't happen or find a way to change them. And they are hardly visible if you have dark themes. Here's an easy way to change them which I used when I was a shared hosting user long ago. Moreover I saw a similar thread today so wrote a quick tutorial.
Open showthread.php (You can find it in your root directory, i.e. where index.php, etc, files are located).
Sorry, but you need at least 25 posts and 5 hours online time to unlock hidden content.
Edit the above colors with your choice and save the file. Now you should have the custom highlighting colors.
Credits to crazy4cs.