The Opera "O Menu", top left is now black. Is that the new normal?
This is normal under the new theme Opera made for Windows 10.
BTW the O and the text turns white in case you set a dark color in your Windows' customization settings and have show system color in the top bar set to enabled in Opera's settings.
In certain cases (if you use a dark theme in 10) Opera is practically unusable.
I believe you're talking about high contrast modes? It doesn't seem to be the case here.