Try to place the custom CSS under the replicated PATH of the theme CSS.
In this case under the root of your project in /assets/
/static/ by default does not accept Assets meant to be used as Resources.
By the way I have never had the need to override a theme’s CSS for a Hugo project, so I do not know whether you will manage to create a single CSS file like this, without messing about in the actual theme’s directory.
So if you manage do let us know.