Let me think of a few themes. Maybe the Theme could be what selects the mecha, not what selects the components afterward. Worse: What if the order of importance between the two is randomly generated, so that it might emphasize the mecha first, then the theme, or the theme first then the mecha, resulting occasionally in Zero Savins with an Anti-Air theme...
"Primal," consisting largely of "natural" weapons such as claws, jaws, spikes and plasma breath.
"Anti-Air," consisting of light missiles, particle beam weapons, and attacks with good accuracy.
"Flight," consisting of the Born to Fly talent, high skill in pilotting, and ballistic weaponry for energy saving purposes.
"Tank," consisting of either Hull Down, Stunt Driver or the pilot bonus to ground huggers, wheels, and artillery weapons.
"Gladiator," consisting of melee weapons and shields and emphasizing pilotting skill over melee attack, so as to get in close.
"Sniper," consisting of high sensor capacity, mid-strength missiles, Spot Weakness, and Stealth. If taken into account in mecha selection, likely to pick Domino and other similar mecha.
Just a few ideas.