View themes-format.md for a reference to the Doppelgänger theme format.
Sample nav bar logo images can be found in images/nav/. If you wish to use these, upload them to web space under your control and reference them as a URL in navbar_logo_image. These should be 450px x 120px.
Note: There is a known bug in build 1341 wherein custom logo images may be displayed incorrectly (shifted to the left) in some cases.
Question? Email me at jcampbell @ scpr.org