Ubuntu 18.04 - how to increase background transparency under modal Gnome environment window?

Question

Graphic shells

The summary of the question is short:

how to increase the transparency of the background or completely remove the transparent background under the modal here:
https://drive.google.com/file/d/1Y2XuHmZnCX-LVpW8W...

Statement of the essence of the question in detail:

I try to formulate the question as clearly as possible and just.
As far as I can find out, my question is about customizing the Gnome Shell theme,
which is installed by default in Ubuntu.

I'm running Ubuntu 18.04 LTS, Gnome version 3.28
(screenshot: https://drive.google.com/file/d/1aZkXhnuxDbVwTeAtx... )

The system is up to date. The default Shell theme is used
(screenshot: https://drive.google.com/file/d/13AN5I0f_F8fZryT0O... ).

Next, I will attach a screen displaying my problem, but at the beginning I will try to describe in words.

Problem:

I use several keyboard layouts, I need Latin and Cyrillic.
The keyboard layout identifier contains the designation of several locales
(screenshot: https://drive.google.com/file/d/1Opo_M5iM3VDx1548_... ).

At the moment when Ubuntu displays a modal window for me on the screen asking me to enter the superuser password, or at the moment when I use the command input window, which can be called by the hot key [Alt + F2], under the modal window you can see a transparent overlay that radially darkens towards the edges of the screen. Thus, the Keyboard Layout ID turns out to be very dark and it is difficult for me to see what information it displays at this moment
(screenshot: https://drive.google.com/file/d/1Y2XuHmZnCX-LVpW8W... ).
(another version of the problem: https://drive.google.com/file/d/1WvJtlHsG9nEBpiIKZ... )

Question:

How can I increase the background transparency of the background (or even remove the background) below the modal
so that I can see the keyboard layout text identifier clearly enough?

As far as I was able to figure out, possible solutions could be:

- using an extension from Gnome Extensions.
- configurators like Dconf (relevant for Gnome 3), Gconf (as it turned out - not relevant for Gnome 3).
- customizing the Gnome theme yourself by editing its CSS file.
- setting the flag instead of the text identifier of the keyboard layout.

My personal results:

- Gnome Extensions: I searched among extensions, but did not find an extension that would provide a solution to my question.
- configurators like Dconf (relevant for Gnome 3), Gconf (as it turned out - not relevant for Gnome 3):
I could not find the desired setting in these configurators.
- customizing the Gnome theme on your own by editing the CSS file:
I couldn't figure out the name of the class that is responsible for the style properties of the element. This is most likely the right way to go about solving the question, but you need to know the specific CSS selector responsible for the transparent background above the desktop.
- setting a flag instead of a text identifier of the keyboard locale:
I found instructions on the Internet on how to do this, but in any case it is important for me to get an answer to my question above: how to increase the transparency of the background or completely remove the transparent background above the desktop under the modal window?