can you help me precisely to understand how can I use this command to change the background color of each widget separately?
In order to do this, you should learn how to find the unique identifier for each widget.
First, make sure you are using an updated web browser such as the ones listed at http://browsehappy.com/
Right click on any widget on your site and select the "Inspect Element" option. Look at the HTML that appears until you find the assigned HTML tag (the one you right-clicked should already be highlighted).
Here is an example of the opening aside HTML tag from the current widget setup on your site:
<aside id="text-3" class="widget widget_text">
The id="text-3" part is a unique identifier for each widget. So, to target just the first text widget in your current sidebar, you would update the CSS from above to something like this:
Add that as a separate CSS rule at the bottom of your Appearance → Themes → Customize → CSS panel.
Change the color code as needed.
Also, since you're really getting into CSS, you should totally check out a CSS tutorial like this one! :-) it will teach you about CSS basics including classes and identifiers and how selectors work, which will come in really handy with the work you have been doing.