Thanks, although I'm not familiar with KSP, do you have a link?
Are you using Cabbage version 2? It doesn't have any logo on the GUI. If you're still using v1, then I suggest you head here and grab the new version. It has quite a few improvements on version 1.
You could create your labels as images or SVG using whatever font you likes, but changing the default font is not possible at present. At the moment, Cabbage ships with its own font, meaning it will display text without an issue even if the font is not present on the system running Cabbage. I'd be Ok with letting users set a particular font file, but I don't like depending on system fonts as they change quite often.
Would a single font be Ok? I could look into adding a new identifier for this, but I think I would like to make it global, one font for all. Making each widget capable of using it's own unique font would be a bit of a pain.