Custom CSS file

Create a file custom/Espo/Custom/Resources/metadata/app/client.json.

{
    "cssList": [
        "__APPEND__",
        "path/to/your/file.css"
    ]
}

Make sure the css file is readable by the webserver. It's recommended to put it in client/custom/css/ or client/custom/modules/{module_name}/css/ directory.

Then clear cache.