Is there a reason that so much whitespace is used on the left-hand side of the user interface? It would probably look better if it were centered.

I think the left bar + middle is centered, and then the TOC is hanging off from the side which throws off the centering. I don't think there's an issue about this, would you mind sending a bug report to fix this? https://github.com/dendronhq/dendron/issues/new/choose

Of course, will do