Wordpress connexion admin error custom CSS

Hi !

On my Wordpress Multisite SAAS, blog owners can login on the main site. When they do that, they are automatically redirected (by Wordpress default behavior) to this page : https://space.zeo.net/g/4680i

I would prefer to directly redirect them to their dashboard but it seems it not possible, according to my other thread : https://premium.wpmudev.org/forums/topic/redirect-user-to-their-primary-site-dashboard-after-login-on-the-main-site#post-698774 ? So, is there a way of customizing the CSS of this error page ? What do you do on edublogs ?

Thanks a lot !