Can the full-screen picture viewer have a white background?

Yes. In your site's CMS, open Theme, then Advanced, then Images, and set Picture Viewer to Light. A picture opened full screen then sits on your page background (white on a white theme) with the caption in your text color. Dark, the default, shows it on a dark screen.

The picture in the viewer has the same corners as the pictures on your pages, set by Rounded Pictures on the same tab. An artist site starts with square corners.