The arrows in the combo boxes like for example at the top of the bugzilla bug report page have the wrong color with the Adwaita gtk3 theme. This is probably a very similar bug to https://bugs.webkit.org/show_bug.cgi?id=120581 and https://bugs.webkit.org/show_bug.cgi?id=118234.
If you compare the combobox more closely to the ones in native gtk apps you can also notice that the padding on the right seems larger in webkit than in gtk natively. Also the arrows are a bit smaller in webkit.
This should be fixed now by the theme rework when using GTK+ >= 3.20