Custom CSS Not Responding

I have removed the WP logo from my login page at

Now I would like to move the login box down to the centre of the page, so have added Custom CSS of
/* Move Login Box Down To Centre of Page */
.login form {
margin-top: 200px !important;

but it isn't having any effect. I assume it is being overwritten somewhere, but I don't know how to fix it. Would appreciate any help.