The theme team is in some ways just a form of cross-support team. However, this form of team has its teammates support each other all in the same way. Rain teams, Sun teams, Sandstorm teams, Hail teams, entry hazard teams, baton pass teams, Trick Room teams, Gravity teams, Tailwind teams- these teams all function differently, but each of them has only one focus- and every team member works in that direction.
For a team of this sort, you run the risk of making your pokemon too easy to pigeonhole- many a Rain team suffers fatally when facing a team with a few good Grass or Electric pokemon in it. To that end, look outside the obvious choices- this is actually easier than you would expect. Keep in mind the theme of your team and what it benefits- Rain helps water moves -and- Thunder, while hindering Solarbeam and fire moves. Hail damages anything not of the Ice type, and pokemon with Ice Body or Snow Cloak become much harder to take out during that weather. Sandstorm doubles the Special Defense of pokemon with Rock typing, damages anything not Rock, Steel, or Ground, and boosts the evasiveness of things with Sand Veil. Sunny Day accelerates Chlorophyll plants and protects Leaf Guard plants in addition to boosting Fire moves and cutting back the power of Water moves- it also makes Solarbeam a single-turn attack. You can take massive advantage of entry hazards if you’ve got Whirlwind or Roar, any pokemon that badly needs a boost loves to be Baton Passed powerups, Trick Rooms make Shuckle hilariously speedy, Gravity grants accuracy to otherwise middling moves like Rock Slide, and Tailwind will relieve you of more speed woes than you can blow a tree down over.
Use this to your advantage- any theme team should have three kinds of pokemon on it- pokemon that set up your theme, pokemon that were already good and get better because of your theme, and pokemon that were only okay and suddenly become great because of your theme. These aren’t necessarily separate pokemon, either.
Keep an eye out for pokemon with stats, abilities, or moves available to them that make these themes beneficial to them. Put Cacturne or Herdier in a sandstorm, Electrike in the rain, Wailord in the hail, and Exeggutor in the sun. Put Arcanine in your entry hazard team and give it Roar, have Drifblim Baton Pass Calm Mind to a Golem, put a slow Rampardos into a Trick Room and watch it tear things up before they get a chance. Throw Stone Edges and Focus Blasts in the Gravity, or shake things up with an Earthquake while fliers and levitation artists can’t get off the ground. And use Tailwind to turn Tyranitar from a terror into swift death, or Ampharos from a plodding lighthouse into a swift beam of doom.
The variety of pokemon able to benefit from any given theme is enormous, making it very easy to keep such a team balanced- as long as you don’t go overboard. Whatever your theme, make sure your team can at least partially function when that theme is stymied, and you will at least force your opponent to spend some of their resources stopping you, thereby leaving you free to play.
