Just wanted to note - I had the same white flashes, when a new page loaded. But, then I opened "about:blank" page, then that page changed color to dark, and now opening new pages there are no more white flashes.

It asked me to create password when I synced my Opera (I created different password than Ubuntu user password).
I think it is a security, because when I sync, it also restores my passwords for web pages (facebook, gmail, other sites, where I saved passwords).
So, maybe, if my family members know my user password, they cannot use my Opera web page saved passwords if they don't know my keyring password(?).