Hello!
Press the key on the top line of keyboard called "f11". It can both enter and exit full screen mode.

Hello,
I have same problem the second day, hope soon it would be fixed...

Thanks for this cool topic guys!
I understand that this is beta, but in some places still need to work,)
On some, the black font changes to white, but the substrate does not change color, making it impossible to read anything (an example of the sinoptik website), but sometimes, on the contrary, dark icons remain black even though the background changes (gmail example).
Good luck in the subsequent corrections and developments!
Here's a link to the album with examples: https://photos.app.goo.gl/JMB9vksR4z56uYtT8