I believe it's called cel shading, and it's only per-map. If you want to shade the entire game, including player models, items etc., you can only do it on your own screen with GLSL shaders.
If you want to cel shade a map, you need to set the worldspawn's _celshader key to the shader you want to use for cel shading. We had cel shaders in Nexuiz but they weren't ported to Xonotic. You can find them
here. You only need to set the worldspawn's _celshader to cel/black_ink for example to get black ink outlines.
I think you can also use q3map2's -celshader parameter, for example -celshader cel/black_ink, without having to change the map file.