css file placeholder for my custom styles for fine-tuning (not heavy theme-ing)

I am trying to convert my plain html site
https://www.starmessagesoftware.com/
to drupal.
I am collecting here http://drupal.starmessagesoftware.com/
the modules I need and want to start a bit of css styling.
- I do not want to start editing the theme's css files because I am not sure I will stay with this theme.
- I do not want to edit the drupal's css files to avoid loosing my changes when I upgrade. (or having to manually save them and reapply them.)

I would expect that drupal had an empty css file, for the webmaster to add his own styling to fine-tune the appearance.
E.g. drupal checks if a file named "yourcustom.css" exists and loads it.
- That css file should be not part of the theme files, so it is preserved during a theme change.
- The custom css file contents should be preservered when doing drupal updates: the file should not be part of the drupal zip files so that it is not ovewritten during an upgrade.

Does such placeholder file exist?
If not, what is the closest approach to this concept?

thanks
Mike