If its just that one style you want to edit, you could try…
add_action('admin_head', 'admin_styles');
function admin_styles() {
echo '<style>
#poststuff #post-body.columns-2 {
margin-right: 300px;
dispay:flex important;
flex-direction:column !important;
}
</style>';
}
Or add a separate stylesheet with something like this…
add_action('admin_head', 'admin_styles');
function admin_styles() {
echo '<link rel="stylesheet" href="https://wordpress.stackexchange.com/questions/314539/style.css" type="text/css" media="all" />';
}