CSS files end with the .css file extension.
CSS files can be created in three different ways in Spaces.
1. Starting with a blank template. This will create a CSS file ready to use.
2. Upload your own .css file
3. Create a new file in the File overview and change its name to something, ending with .css
You can read about changing file names in this related article: How do I rename files and folders?
You have to enter code into the file to make it functional.