Do your theme yourself

I’ve read an article about how the Haiku’s icons can be so light. That leads me to the differents features that usual OSs provide to customize the appearance.
I conclude that if we can change the desktop color or set a wallpaper, that can be inconsistent with the icon and texte color, the window color and everything upon the desktop. So if we can change the desktop background, we should be able to change the rest accordingly.

Can I do that with Haiku?

I’ll go further. Is there a GUI toolkit that the developer use and which could be available to the user as well?