What are the options for using child themes in WordPress?

Question

WordPress

Explain, kind people, how to make adjustments to a child theme and what restrictions exist (if any).
I need to make adjustments to the responsiveness settings. On the main theme, everything changes easily and works, but the theme is premium, its updates are periodically released, respectively, after installing the updates, the files are overwritten and the settings are reset. So there is a need to use a child theme.
The task is as follows:
there is a responsive.css file
, it has a line

@media screen and (max-width:768px) {
 ...
}

I need to change the value in pixels in this line to the following

@media screen and (max-width:900px) {
 ...
}

How can I do this in a child theme?
At the same time, the responsive.css file is located in the css folder
Accordingly, what I did: I
created a css folder in the folder with the child theme. Inside this folder, I created a responsive.css file, that is, exactly how it looks in the main theme. In responsive.css I included the file from the main theme and added changes to the media query string, the result is the following:

/*
Theme Name: Travelista Child
Description: Child theme for the Travelista WordPress theme
Author: Simrandeep Singh
Template: travelista
*/

@import url("../travelista/css/responsive.css");

/* Start Adding Your CSS Here */
@media screen and (max-width:900px) {
...
}

Nothing works, perhaps this is not surprising. Tell me what I did wrong?