The CSS import feature is trying to validate the CSS file and refuses to continue if the file is not matching the known syntax, error:
"Import is aborted. The provided file does not have correct CSS format"
Aria Automation 8.18.x
Aria Automation is configured to validate CSS according to CSS Syntax Module Level 3, see Edit and Version a Custom Form in Your Cloud Template which does not cover newer CSS concepts such as variables.
This is a current feature limitation, a feature request has been opened to allow support in a future version.