This has actually been around for many versions - when a tab is colored white, the 'x' to close the tab is no longer visible as it has no contrast with the tab itself:
Hello,
Unfortunately we don't have the control over the X color with the third party. I already tried to find a solution with no success. It's on my know issue list. If I remember correctly, you can set the color to nearly white but not white and it works better.
Regards
David Hervieux
Thanks David.