I would like to learn how to write a CSS class without constantly asking for your help, as long as this does not mean revealing your professional "secrets" acquired in your professional activity.
Where this information (.bg-group-background) can be found?
and again, I would like to be able to change the background color and size of the text of the list of dashboard...
Then you can play with applied CSS rules to see what changes if you alter the CSS rules. You can turn on an off current rules and you can add your rules to some classes right inside the styles tab.
Those changes are temporary. If you find what you want that way, you can write your own classes to apply or override current rules.
From this point everything about the CSS gets complicated but adding and removing classes or rules is as easy as you already know.