Well due to the other issue (bug in SPPB) now my site is down and I am extremely frustrated.
I do not what kind of screenshot you expect. The issue is serious but very simple.

I set page background to i.e green, but when I open the pages it is white everywhere. That;s all there is to it.

For example: now I have a need to setup articles background (I mean standard J4 articles).

Hi Arthur. There are no problems with Helix (if we are talking about raw Helix Ultimate, but not about commercial templates based on Helix). I have been using Helix framework for more than 5 years without any critical problems. There are small minor disadvantages that are easy to fix. But there are similar things in any software.

As for the use of custom CSS coding, in this aspect, Helix works excellent and is the most convenient of all existing in the market.

Problems can be local in nature, which depend on your server, your skills and your understanding of how software works.

For the background of articles pages, use the following code

.view-article {
    background-color: red;
}

For the background of articles content part, use the following code

.view-article .article-details {
    background-color: white;
}

Add it to the custom.css FILE, but not to the templates Custom CSS field or somewhere else.

If you think that your code is correct but does not work, you should use browser dev tools to determine the cause. Or write a code directly in dev tools to check its work capacity.

Tip