Custom CSS for WP Network Admin confirmation pages, drop it into mu-plugins or a plugins folder or adapt it to a themes functions.php. The css file is meant as an example to override the WordPress logo with a custom one.

bc-confirm-css.css

.wp-core-ui h1#logo > a
{

    border:1px solid blue;
    /**
    * change all the overrides here
 
    width:;
    height:;
    background-image: none, url(/path/to/url.image);
    background-size:;

    */
}

bc-confirm-css.php

<?php
/*
 * Plugin Name: WP Admin Network Confirmation Screen CSS
 * Plugin URI: https://gist.github.com/bearcodi/9478758
 * Version: 0.1.1
 * Author: Mark Babic <mark@bearcodi.com>
 * Description: Custom CSS for WordPress Multisite Network Admin confirmation pages
 * License: GPL2+
 */

namespace Bearcodi;

/**
 * Add an action to add CSS to change the WordPress logo on the confirmation screens in WP Admin
 * @return [bool] Returns true if we cant get_current_screen()
 */

function confirmation_screen_css_action() {
//  Make sure we can use the get_current_screen object
    if( !function_exists( 'get_current_screen' ) ) {
        return true;
    }

//  Get the current screen object   
    $screen = get_current_screen();
//  Retreive the confirmation action, it is actually action we are targeting
    $action = isset( $_REQUEST['action'] ) ? esc_attr( $_REQUEST['action'] ) : false;

//  Accepted screens to insert the logo override
    $accepted_screens = array( 'sites-network' );
//  Accepted actions
    $accepted_actions = array( 'confirm' );

    if( in_array( $screen->base, $accepted_screens ) && in_array( $action, $accepted_actions ) )
    {
    //  Uber hack in place here, esc_html is called to display the message in the page
    //  essentialy this is hijacking it to 
        add_filter( 'esc_html','\\Bearcodi\confirmation_screen_css' );
    }
 
}

add_action( 'wpmuadminedit', '\\Bearcodi\confirmation_screen_css_action' );

/**
 * Funtion to tack onto esc_html via filter hook to add our override CSS to the screen
 * @param  [type] $html HTML source in the filter chain
 * @return [type]       HTML with our hacked on CSS file
 */

function confirmation_screen_css( $html )
{
//  Change the path to this override css file to whatever you want
    $css_overide = '/wp-content/mu-plugins/bc-confirm-css.css';
    
    $custom = sprintf( '<style type="text/css">@import url(%s);</style>', $css_overide );
    return $html . $custom; 
}