I understand that it is very hard to make happy all Opera users. I found this thread where someone demands to dev team to make present behaviour. It is mean black sidebar only with black theme and light sidebar only with the light theme. Now, when it is done someone of us wants to return back origin colour system.
So I solved it that I downgrade my Opera to 58 version and I am satisfied.

I really like this design ↓.

@leocg About yellow submarine... Sorry. We all are talking about the same thing. We want to black sidebar back. I want to just know if our request is accepted, denied or you will discuss it in your dev team. Thanks

@leocg Do you have some information for us?

I strongly agree that the present background colour system is not good. The black background of the side bar was much better.

Now it is possible by "drag and drop" of extension icon that you want to replace.