From the screenshots it looks like this toolkit is meant to be used in applications where a non-standard interface is the standard. For some reason this seems to be the case in a lot of audio applications.
Audio apps tend to need to have special UIs and widgets which aren't typical needs for most apps. You need special sliders, drawing tools, and color schemes for live performance that aren't baked into many widget toolkits. Even if they are, you're dealing on such microscopic levels of detail on these UIs that you really do want them to be the same pixel-for-pixel on any machine. And if you're selling a cross-platform audio plugin to musicians for a couple hundred dollars for the last 5 years, it's nice to totally remove OS-specific UI weirdness from the equation.
