Why don't Opera let me change the default search engine?
Opera devs specifically removed this ability from Chromium.

Each browser allows you to add custom search engine and make it default. But not Opera. Opera has removed the ability to do it.

Just compare this with Brave's private tab design:

It looks well.

What about this way?

I wanna have an ability to resize objects only on Speed-Dial too, without a side effect on Bookmarks.

OK, to put it simply:

I can disable "My Flow", "Opera Ads Blocker" address bar icons from the settings. I know it.
I CAN'T disable "Screenshots" and "Bookmarks" address bar icons.

I need the ability to turn off Screenshots and Bookmarks icons.

"Screenshots" and "Bookmarks" icons should behave like "My Flow", "Opera Ads Blocker" icons (They should have the same behave).

Firefox allows to do this.

And one more about the address bar icons:
https://forums.opera.com/topic/31749/add-padding-for-address-bar-buttons-opera-60

Please, add a function to disable default Address bar icons (Screenshots and Bookmarks), like "My Flow", "Opera Ads Blocker".

Yes, it's good functions for some people, but not for all.
I do not need Screenshots button, I use a OS's screenshots program that works on whole OS, not only in the web page.
Some people use other Bookmarks extension, and they do not need your Bookmarks icon.

I want to have the clear interface without unneeded icons.

@nnq2603 Just look at your screenshot and look at that Opera UI team did. ShadowFox is another good example of a dark theme implementation.

It's OK that the dark theme affects all UI.

But I would prefer an option that makes the dark theme affect only Sidebar*, Address bar and Tabs. The old version of Opera works almost the same way.
This approach makes working with bright sites comfortable.

*And allow use Sidebar only on the browsers pages (Speed-dial, Bookmars, etc), like it was in Opera 58. I do not need it on common web pages.

As I said, most sites have a light theme. The old dark theme affected the upper part of the browser and Sidebar (Settings too, but this is the rarely used part of the browser). Switching between sites and browser pages (bookmarks, downloads, history) did not cause discomfort. Light context menu is also well suited to the overwhelming number of sites. Dark side panels did not distract attention when viewing light sites.

Now the whole interface has not just a dark theme, but a black theme. And this theme is too contrasting, bright white text on a practically black background. I suppose the designer decided to copy not the best design of the dark themes of Windows 10 Explorer.

A really good implementation of the dark theme is Darcula (Intellij IDEA), why not copy it? Or Steam design (Valve).

Screenshots of sites on a black background of Bookmarks, the black contrast context menu and the list of recent downloads look foreign on ligth themed sites. Switching between sites and browser pages looks like flash light.

At the same time, the colors of the dark themes of Address Bar and Tabs are not combined with the colors of the dark themes of the sites. Why are you using a dark blue-green color?!

By the way, Windows 10 from the beginning has a dark Taskbar with a light theme, just like it used to be in Opera, and it looks good.

Notice the contrast of the context menu. Opera's context menu is too contrast. The other things too. It is a fact.
Opera context menu:

Windows 10 Taskbar context menu: