The only real theme is that your game contains Mech / Mecha.

Then there is the optional secondary themes. Remember these are optional secondary themes. You can use as many or as little as you like (even none at all), the only fundamental rule is that your game contains mech/mecha. The optional secondary themes do not impact the voting in any form.

Diplomat 👔

Time Travel ⌚

Weird Fiction 📖

Arena 🏟️

Shifting Sands 🏜️

Cosmonaut 🧑‍🚀

Relax 💆🏻‍♀️

Signal Failure 📡

Resurrection 😇

If you do use any of these (remember you don't have to if you don't want to) then you are free to interpret them however you want.