Honestly, it’s way more convoluted and frustrating than it has any right to be. The only tools I found were cursor-toolbox which allows you to convert SVG templates to the correct set of PNGs and xcursorgen which converts the PNGs to actual cursor files. It took me several tries just get a working cursor set. Then I spent much much longer actually drawing and tweaking my theme using inkscape. It was certainly rewarding to get it working though. Now I smile every time I see the little “busy” animation.