Without png support in imlib2 buttons and window borders in e16 are missing. The bug was discovered on a relative fresh installed system on ARM architecture but I think that doesn't matter. I guess the jpeg use flag might be required too.
while it's true the default theme utilizes png images and so building imlib2 w/out png support will mean things do not render correctly, there's nothing stopping you from loading a different theme that doesn't use png images.