First, let me introduce you to stylus. It is an extension for pretty much all browsers that allows you to write your own CSS for a website of your choice. You can install other styles which people made or you can write one yourself.
Making stylus themes is pretty easy, you can just take the classes of elements on the website you want to make a style for. But there is an exception, websites that use JS frameworks such as React have changing CSS classes and that means your theme, over time, will probably stop working.
There is also a possibility of adding settings to your themes, tho I haven’t messed with that yet.
But nonetheless, making themes is really easy.