SublimeText uses "Skia Graphics Library" to draw the entire user interface.

However, you still need to write Cocoa for macOS components and GTK for Linux.

Many cross-platform programs do the same, developers use a high-level library to render the overall interface, but underneath they are still using the native graphics libraries offered by the operating system where the program is running in, in this example, macOS and Linux. You can still do a lot with Skia alone, but eventually you'll find yourself writing some platform-specific custom code.