I personally consider the Pangoro theme deck called "Dark Hammer" (came out alongside the Furious Fists expansion) to be the best currently available, as it has good attackers, good support Pokemon, and a useful selection of Trainer cards. It's an older deck though, so it might be harder to find in stores.
I also like Stone Heart (featuring Regirock; came out with Ancient Origins) because Regirock is a pretty hard-hitting Basic, plus it has Poison shenanigans with Ariados. Only downside is that it doesn't have the best draw support. It also comes with 3x Combee which can help you out in the future if you get interested in running a Vespiquen deck.
Though, while it's true some theme decks are better than others, since you are a beginner and not too interested it competitive quite yet I'd honestly suggest just picking up a theme deck that features a Pokemon you like! Or better yet, buy two and create your own deck from the various cards.
None of the theme decks will be particularly viable at Leagues (unless there is a Theme Deck tournament, of course, but those are quite rare), so no reason to fret too much over whether one is on that level or not.
As for trying them out, there is a way, but it'll take some time -- the Pokemon Trading Card Game Online (free to play for PC and mobile) has most XY-era theme decks available in their store that can be bought with in-game currency. It'll come down to accumulating enough coins to purchase them, either by completing matches, logging in every day, achievements, and so on. Plus, PTCGO is great for practice in general, so that's definitely something I suggest you look into as a fledgling trainer.
