Learning to be competent in C++ might be overkill, unless you also just want to acquire a valuable skill. You might try just messing around with the code before receiving your keyboard. If you haven’t changed much and the code compiles still, it might be right. I worked out my layout a month before I got mine.
But aside from the needs of skill in C++ in the firmware, it’s also a great thing to learn, anyway. That, and you really will need at least some understanding when you inevitably run into compilation errors. I recommend Bjarne Stroustrup’s A Tour of C++. He’s actually the guy who originally invented the language, and he’s still very active in the community and standards board. This book is better than many you’ll find, because a lot of them don’t teach best practices. It’s also a lot thinner than his authoritative The C++ Programming Language.
Finally, keep in mind that the Arduino implementation of C++ has a couple differences. There’s a good discussion about that in this thread.