I dont't have access to all my installed fonts in design template settings. I can access all fonts in the mindmap editor. But some fonts are not available when I want to make them default in my custom design template.
I tink the difference is that the normal Map Editor is using an updated font selection dialog which also allow OpenType fonts (the ones with the 'O' icon) next to the TrueType fonts (the ones with the 'TT' icon), the Map Theme Editor still uses an older font selection dialog which doesn't seem to show the OpenType fonts.
Ary's work around might work for Map Templates (the map structure as a template), but if you're looking to create Map Themes which also include specific information to allow new topics/relationships/floating topics etc. to be created using specific font, color etc. then I don't think there is a work around here?