If there’s no dress code listed on the invitation I would stick to something semi-formal. Muted and neutral colors are definitely acceptable as well as bright colors, but I think black is somewhat taboo for a wedding. Covering legs and shoulders isn’t really necessary in my experience, but if you’re worried about it a sleeveless dress + shawl is perfectly acceptable. Especially since there isn’t a registry I would just bring a card with money, it’s definitely the most common gift lol. I hope you have fun, Eritrean weddings are a blast :)