Whilst I don't wish to be engaged/married/having a wedding reception at Alton Towers under any circumstances (or at any theme park for that matter), I do believe that there are several venues around the resort that can be used for such events.
As Ben said above, the chapel would be excellent as a venue for a wedding because it has the space and is essentially an extension of another religious institution like a church, but so would Her Ladyship's Gardens for a small ceremony or even the main Gardens themselves, such as on the lawn near the Earl's bust or the Conservatories. I wouldn't hold an actual ceremony inside the Towers because that would be too much of a H&S risk, no toilets/heating/roof/allowance for guests with mobility issues and... well, pigeons, as much as it would be gorgeous to use some of the huge rooms.
For reception venues like a small party, the Swiss Cottage seems to be a great tucked away location. Towers Bar and Grill wouldn't do too badly either if you were to clear all the tables and chairs away.
Then of course overall you have the Conference Centre between ATH and Splash which seems like the best place for either event really because of the various rooms and the bar area just after the first set of doors. It would also mean less faff when getting to the venue because it means just getting the walkway.
As for themed weddings and receptions, it would honestly depend on the couple. If they were enthusiasts like us, I really wouldn't put it past them to try a theme around one of their favourite park roller coasters (the majority's top favourites being Nemesis and The Smiler right now) or an area theme, even ATH's theme would do well as it's fairly straightforward.