Wrong background behind menu tab corners in red and purple colour schemes

If I select either the red or purple colour theme, the rounded corners of the menu tabs show the wrong colour behind them - instead of the page's actual background colour, there's a pale blue-grey which I think may be the background from the default vanilla colour scheme. This is particularly jarring on the red colour scheme since it's got dark red tabs on a bright red background, with these little grey corners standing out like a sore thumb. It's not quite as obvious in the purple colour scheme, because the colour of the tabs is lighter, but it does make them look like they're rectangular rather than rounded.

This happens in both Firefox and Safari. Is there any way to correct it, maybe with custom settings or something?