zulip/web/styles/scheduled_messages.css

#scheduled_messages_overlay_container {
    .scheduled-messages-loading {
        margin-top: 10px;
        display: grid;

        > * {
            grid-row-start: 1;
            grid-column-start: 1;
            margin: auto;
        }
    }

    .no-overlay-messages {
        display: none;
    }
}