invoiceninja/config/css-inliner.php

<?php

return [

    /*
    |--------------------------------------------------------------------------
    | Css Files
    |--------------------------------------------------------------------------
    |
    | Css file of your style for your emails
    | The content of these files will be added directly into the inliner
    | Use absolute paths, ie. public_path('css/main.css')
    |
    */

    'css-files' => [
        public_path('css/app.css'),
    ],

];