How to change the white background color when loading pages?

Question

User interface

Hello. First, a little introduction:
I work in a dimly lit room and as a result I can't stand bright white backgrounds. That's why I use the Stylish extension https://addons.opera.com/en/extensions/details/stylish/ . This extension allows you to install themes (dark in particular) on various sites that the good people of this world post and update on the site https://userstyles.org/ .
I myself use Opera, but the problem described below applies to all browsers without exception.
The essence of the problem:
When navigating between two sites, a white "loading" screen appears, which hurts your eyes if you navigate between sites with dark backgrounds. This white screen is always there, but it is not noticeable if you work at a computer in a well-lit room and without dark themes installed on sites. That is, if the site you are on is white, the loading screen is white and the site you go to is also white. If the backgrounds of both pages are dark, then the white background of the "loading" page will hit you in the truest sense of the word. Over the past year, I've gotten the hang of closing or lowering my eyes as I navigate between sites, but this has finally worn me down.
Steps to reproduce the problem (this is an example, does not have to be these sites):
1). Go on the websitehttps://translate.google.com/ , with theme preinstalled on it: https://userstyles.org/styles/134994/dark-google-t...
2). Then go to the site https://en.wikipedia.org/wiki/White , with the theme pre-installed on it: https://userstyles.org/styles/42313/dark-wikipedia
This problem is also shown in this video: https: //www.youtube.com/watch?v=HfJ_EwSTevI
What I want to do:
Change the color of this white "loading" page to dark grey/black. I note that if the page is cached, then this background does not appear, which is natural.
I also note that on some sites the color of the "loading" page has been changed by the creators of the site to dark (that is, the problem is solvable). And also the fact that I found solutions to this problem, but did not know how to apply them. For example here: https://superuser.com/questions/580228/prevent-whi... . Even omitting the fact that this solution apparently no longer works, I still don't know what to do with these codes. Following the given "instructions" did not lead me to anything.
I can't find a solution to this problem for a very long time. In all the forums on which I wrote earlier, the answer was not found. I suspect that the problem is that people did not understand what needed to be done, as a result of which I painted this question in such a detailed manner this time.
Please turn off the lights/draw the curtains in your room and follow steps 1 and 2 to experience first hand how relevant this problem can be.
Thanks in advance.

Answer 1

I don’t know how it is in Opera, but in Chrome I have the Material Dark theme and the white background of the “loading” page has changed to black.

Answer 2

I think I found a solution, but it's for Firefox: https://userstyles.org/styles/141761/dark-loading-...
Can anyone explain how to do the same for Opera?
And here's another one: https://userstyles.org/styles/103659/deepdark-abou...

Answer 3

f.lux will help smooth out the brightness anywhere

Answer 4

Here's how I fight: https://www.youtube.com/playlist?list=PLIAheh4XV1_...