ezTOC adds a span tag to the header. This span tag is what provides the link “target” for the TOC. I suspect the way the theme designers targeted the header with CSS for the color that the added span tag make the selectort invalid.
I would not classify this as a bug but is a conflict. If you know some basic CSS you could change the header to any color you wish, if not, you might have to decide which is more important, grey’ish headers or a TOC.
Not sure this helps, but I hope it does.