In the gallery view, SmugMug appears to only show a few words of a caption by each thumbnail. This is almost a deal-breaker for me for SmugMug as a whole. I'd like my entire titles to show, even when they are somewhat long. Can this be done with a custom theme?
That depends on the gallery style you use.
Thumbnails gallery style only shows a portion for longer captions below each thumbnail. I don't think that can be customized to show the full caption as captions on SmugMug can be up to 65,000 characters and that would not fit with the layout of the thumbnails style. Click on any thumbnail to view the photo large with the whole caption.
SmugMug style does show a main photo on the right below which you'll see the full caption. Click on any thumbnail on the left to see the photo large on the right side along with the caption.
Journal style shows the full thumbnail below each photo.
You can change the style in a gallery via the gallery settings > look&feel > style setting. Give the different styles a try to see which one suits your needs best.