display: flexbox; and justify-content: center; in CSS is a good way to center the login structure. Read this post for more info.

Code:

    .form-container {
        display: flexbox;
        justify-content: center;
        // add rest of styling
    }

Warning! Below Deprecated!
The center tag can be used to center the text horizontally. The downside to using this is that it is deprecated and may be removed in future versions of HTML.