Those are beautiful inspiration pictures! You don't really need a theme, though if you want one, go for it! My fiance and I never picked a theme, though as we were buying a lot of our decor and smaller items (guest book, vow books, centerpieces, etc), we realized that our decor seemed to follow a forest/nature theme, so that's our unofficial theme, lol. Look for things that you like, not just things that fit the theme. After a while, you might realize that your decor follows a theme you hadn't thought of yet.