Carbon is described as 'You know all of those code screenshots you see on Twitter? Although the code's usually impressive, we saw room for improvement in the aesthetic department. Carbon is the easiest way to create beautiful images of your source code. So what are you waiting for?' and is an website in the development category. There are more than 25 alternatives to Carbon, not only websites but also apps for a variety of platforms, including Mac, Windows, Linux and SaaS apps. The best Carbon alternative is ShareX, which is both free and Open Source. Other great sites and apps similar to Carbon are Greenshot, GitHub Gist, Karbonized and Ray.so.
You can't customize anything outside themes.