HTML is a markup language. This is important to format your web page
CSS is styling and designing for the page what you have wrote using HTML
PHP is the server side scripting language WordPress built on

Additionally
MySQL is a database to store all your WordPress data

So I would suggest you to get hands on all these for if not so master than at least basic knowledge will helps you lot.

WordPress has own built in function for Theme design which well known as Template Tags.

So if you know bit of HTML, CSS and PHP than you can make basic to mid-level WordPress theme. However, to make highly advanced theme like on marketplace, you should have advance knowledge of PHP and MySQL too.

Also for WordPress plugin development you need great knowledge of PHP and MySQL.

In fact to be a great WordPress developer you should have good knowledge of WordPress APIs and Codex too. This is how you will work with WordPress architecture.

Great thing is, WordPress have great documentation for Codex and APIs