This is a documented issue and near 4K users reach a comment in Operaforums wich seems to be ignored!

Many users suffer from visual sensibility, specially gamers that plays many hours in dark rooms, it's very disturbing get flashed every time you open a new window or a new tab with a white background.

And it seems very easy to fix it:
https://github.com/hbtlabs/chromium-white-flash-fix/commit/b1c4d8efd43fa46c61b9dc695f387f41a16947fb

Please try to fix it as soon as possible it's hard to bear.

Thanks