Short answer is that any work with a +2 next to it on the drop down list is currently contributing to a robust theming bonus. Focus on the +0s, but before doing any of that tinker with your slot assignments. There may be an overlooked theming bonus combination (perhaps only yielding a +1, but that's OK early on). Focus initially on trading up your +0 bonus items, but always have a specific profile for your desired swap item. Affter you've addressed the +0s, move to the +1s (e.g., museum wants same civ and same era, and you have an Assyrian Ancient artifact (say, beads) and an Assyrian Classical artifact (coins) = +1, but swapping the +1 coins for another Assyrian Ancient artifact (more beads would be OK) will get you to +2).