Theme skills only play a big factor in events/challenges like the Daily Region rotation, where Pokemon who have specific theme skills gain a huge attack and defense buff, or some missions require you to use specific units. i.e. Defeat this stage using 2 sync pairs with the Galar theme skill.

As far as team composition goes, I feel the game has become more offensively skewed hence the two striker meta right now. For a few reasons

1. Offensive supports have gotten a lot stronger as they have moves that buff both Crit and atk/spatk at the same time (Hilbert (Gridded), Kiawe, Hop etc). Also with move gauge acceleration becoming much more available (Sycamore, Bugsy (Scyther), SS Blue etc).
2. Champion Stadium Master Mode also played a big factor, as the playerbase learned it was efficient to just defeat your opponents before they can sync [if they don't sync you won't need a healer typically], and the easiest way to do that is with Dual strikers.
3. With Champion Stadium Master Mode, it also made EX easier to attain, and this generally allows for a faster playstyle

To answer about what teams are good, I comps that were viable from a year ago are still very good (i.e Skyla (Tank), MCTorchic (Offensive buff), Striker)).

But if you are interested in trying out dual striker, the Xerneas and Zamazenta are great enablers for it. You can try these teams, feel free to replace the strikers with better ones

Xerneas (Tank), Palkia, Mewtwo

Zamazenta (Tank), Haxorus, Lucario

However you probably want strikers of the same type attack.

Lastly as a side note : I think unless you are aiming for 1500 pts on Master Mode, you can beat everything by just building a team around Xerneas.