For the benefit of you and this book, the following assumptions have been made with regards to the skills required to edit and create themes for Moodle.
You will need to be reasonably familiar with HTML and CSS and have some exposure to graphics manipulation software packages. By any means, you do not have to be an expert, as Moodle theming is quite simple as long as you follow the instructions carefully. So if you have built websites before by using HTML and CSS, then you should hit the ground running. If you have only dabbled in web design before, you still shouldn't have any problems with the exercises in this book. If you are a total beginner, you can still do this. All you need to know is that you will need to take your time and make sure that you understand everything that you are learning before moving on through the book.