- to make a copy of your edited css file
- rename it (I use custom.css)
- and edit the Joomla template index.php file to load that file as the last event in the head area.
This will then override the new vmsite-ltr.css that will be created in an upgrade, and any fine tuning can be made within the custom.css file.
I use a custom.css file in just about every Joomla site I build.