Ah damnit! It looks like something I have downloaded/installed used bash syntax in the post-install (or post-uninstall) scripts (incompatible with the syntax of the ash shell which is built into busybox).

I cured my problem by reflashing the OS (there was a newer version available anyway), but I'll install bash anyways - I'm feeling lost with the limited functionality of ash. But my last impression before reflashing was that some package just removed the image files for all themes from /usr/share/themes or whatever... so there were simply no images for button boundaries, scrollbars and alike...