• Follow https://docs.cloudron.io/apps/guacamole/#branding to first install the extension
• Unfortunately, my css skills are not great. But I put this in css/example.css :

.login-ui .sso-providers:last-child {
    display: table-row !important;
    text-align: center !important;
    vertical-align: middle !important;
    position: initial !important;
    height: 30%;
}   

• Then, I reuploaded the jar

To explain the css a bit: it seems that the element is by default absolutely positioned at the bottom inside a table layout ! (go figure)

You can see similar screenshot at https://theko2fi.github.io/apache guacamole/2023/03/26/apache-guacamole-single-sign-on-integration-using-openid-connect.html as well . Maybe you can report this upstream?

(The big login is centered on the screen.) Because none of my customers using it can even find the openid login