.wp-block-memberpress-thank-you-success {
    border: 1px dashed blue;
    padding: 50px;
}

// Ensure editor wrapper divs respect the flex layout
.mepr-success {

    >.block-editor-block-list__layout>.wp-block,
    >.block-editor-inner-blocks>.block-editor-block-list__layout>.wp-block {
        flex: 1;
        min-width: 0;
    }
}