CSS to center the meeting details modal

The meeting details modal isn't centered when it pops up.

This css seems to work well. But when I try to add it to our Crouton custom css, it doesn't work because it only applies it to .modal-content and not #meeting_modal .modal-content.

```
#meeting_modal .modal-content {
top: 50%;
left: 50%;
transform: translate(-50%, -50%);
width: 65vw;
max-height: 85vh;
overflow-y: auto;
}
```