This is one from my fix list as the default highlighted text colour on my theme was green. A quick Google search showed me the way.
To make the change, I had to add the following to my theme’s stylesheet.
There are many WordPress plugins that will allow you to change this. Some will also give you an option within the text editor so you can have different colours per line etc.
Remember to make all changes on a test system first, like a child theme or local MAMP setup. I’m no expert, but the above worked fine for me!