Location of custom.css

Yet I've done all changes via sass or directly into the template.css.
When activating sass, all changes in the template.css of course are gone.
Where can I set my custom.css file, as override?

Hello,

i found this in the manual:

https://learn.getgrav.org/themes/theme-tutorial

Goto Step 3 "Theme Basics". There is a pic of the file structure...

c u!

and there too

https://learn.getgrav.org/themes/customization