The perfect theme does not exist.
There are themes with few options to customize it, others with tons of options. But there will be always that particular string in that specific page that you need to make blue! Why? I don’t care, I love blue, so it must be blue!
What happens if you don’t know how to change it? You ask for support on a forum, on wordpress.org or on the theme author’s support desk.
Somebody gives you the code needed to make the tweak and says to add it in your custom CSS field or file… custom what?
Well, today you will learn what custom CSS is and how to use it in WordPress.