A bit complicated to do "step-by-step". The
www.photo.gallery websites has a lot of custom styling applied, and I am not quite sure what you mean "change a link in the menu". Apart from using the intro-image plugin on all pages at
www.photo.gallery, there is not too much modifications to the X3 menu. We have outlined the "buy" button with a white border. Perhaps you can elaborate?
The best tool to use if you want to start dabbling with custom CSS, is the browser Developer Tools. For example here in Google Chrome. It allows you to click an element, and see what classes and styles are applied to it, which gives an idea of what CSS you could add to custom CSS.