starlight And just to make sure, all the changes we're applying, they will remain when joomla is updated again, right?
In respect to your specific query, The CSS has been added under local parent folder which primary purpose is to isolate customizations and settings. For more details, You can view this
Yet, It is highly suggested to take backup of your site before performing updates, in case rollback required whilst facing any issues e.t.c and to avoid any unforeseen circumstances and also, Ever since, the template has been customized earlier to quite good extent with several changes in files/folders and there has been custom coding involved in order to get the responsive feature
You may review now as I have added certain media query CSS to get the elements right for certain breakpoint