A custom PC case is simply a modified or specially built case for you! This doesn’t always have to involve complicated cutting, windowing, or similar. Customizing your case simply involves making it look the way you want it to look. From simple things like adding lighting to more complex design projects (like the guys over at Caselabs do), what you want done to your case is entirely up to you.
To a large degree, a lot of the newer case designs put out by the top manufacturers (NZXT, Corsair, Fractal, etc) have a more ‘custom’ or ‘cool’ look to them. Originally computer cases were simply a dull beige or black box with standard bays & expansion slots all configured the same way. Nowadays cases can come directly from the manufacturer in a design you may already love. From clear sidings to windows that let you peer into your build anytime you want – some of the culture of the mod community and custom case builders has crept into the standard manufacturing and design of cases that come pre-made. Is that a truly custom case? In the sense that it varies from the traditional boring case, YES! In the sense that the design is unique to you – not really because you’re buying something that thousands of other people have. However, if it’s what you like, then that’s all that matters.