When I consider decks to build I look at them from a Lore perspective and a set Mechanic perspective. In the MTG multiverse what is this deck or guild suppose to do, and from the set mechanic perspective....well that one's kinda obvious. Really it's feeling out the set and asking yourself "what does the deck need", rather than just adding cards that you think would work for it. For example, if I'm running a Rakdos aggro and burn deck I'd definitely add
splatter thug to the mix because first strike is
dangerous, but something that he (the card) would benefit most would be to play
madcap skills. He goes from being a (2/2) to (5/2), still retains his first strike ability, and now has two targets that he can
decimate during combat. Play
titan's strength on him at just the right time and he'll go from being (5/2) to ((8/3) in a heartbeat. That's card advantage and effectiveness right there. Similar to your Boros Theros deck, look at the colors you're playing, look at the set that you're playing, see how those colors were INTENDED to be played and build your deck how you need to. It takes practice for some of the older sets but the same principle applies to no matter what game you play.