Hello @supergrower,
The CSS provided in earlier reply places the media in center position as well by giving width of 100%.
You can try adding below CSS code to your Custom CSS tab under rtMedia admin settings :
.rtmedia-single-container .rtm-lightbox-container .rtmedia-single-meta {
display:none;
}
#rtmedia-single-media-container {
width: 100%;
}
We have checked the CSS again and it is working fine. You can see the same in below screenshot :
If the issue still persists, please once try with any default theme of WordPress and deactivating other plugins to avoid any conflicts.
Thank you.