I think the 'generic' theme could be based on this Athenian example. Just remove all of the Athenian-specific things, like the owls, anchors, and ocean waves. That way, when a modder adds a new civ or the base game adds a new civ, the new layout will still work without necessitating a whole new theme right away. The theme for the new civ can be created later and will be helped with the fact that there will be a dozen examples of how it's done.
I really like how the bonuses are represented by stencils, while the 'entities' such as technologies and buildings are represented by simplified, aesthetically pleasing round icons.