Howto disable gecko's GTK themeing?

Hi,

Because of GTK's inability to act as a theme provider for applications
which require transparency information,
gecko does ugly hacks, which cause a lot of context switches and GPU stalls.
On sites with many GTK themed elements (buttons, input fields,
scrollbars), I get a noticeable hit in painting performance.

Is there a way to disable gecko's GTK themeing, and instead let it
paint something plain & simple?

Thank you in advance, Clemens