Are you in the US or the UK? The options are somewhat different.
First, a Plan B for times when the home automation system is not working
If you are in the US, you will probably need to put one wall switch in just so you have a way of turning the room lights on and off if your home automation system is not working. You can plan not to use that Switch for everyday use in any of several ways, most commonly by just putting a child safety lock on it, or by making it recessed with a cover over it, or by putting a smart switch cover over it which is a battery operated device that has its own buttons, thus allowing you to turn the lights on and off without actually cutting power to the smart bulbs.
But in particular if your theater room is at the bottom of stairs, you will want some kind of manual switch available for emergencies, and you may be required by local safety codes to have that. (If you are in the UK, you will be required to have a switch in almost all situations.)
The following FAQ discusses some of the various switch options, including the smart switch cover.
now back to plan a: the tablet controller
So once you have your Plan B switch somewhere in the room, preferably close to the base of the stairs if there are stairs, then you can go ahead and design your plan A tablet controllers. 
Many people do this, and there are a lot of different ways to do it. Probably the most popular is to use the third-party dashboard app, action tiles. You will have to pay around a $30 license fee per smartthings hub, but what you’ve done that you can install it on as many different devices as you like, including phones and tablets. It just runs in a browser. You can customize which devices show on which tablet and some of the other choices like color and tile size.
(SmartTiles was the early free version of action tiles, and you will see a lot of references to it in the community, but that version is no longer available.)
There’s even a thread in this forum where people describe the different kinds of tablet mounting options they are using with action tiles. Most of these would work with any dashboard option
There are some other free dashboard options which are worth taking a look at. You can find those on the quick browse lists in the community – created wiki in the project report section on the list for “dashboards.” But I think most people end up choosing ActionTiles.
One other option: a Button Controller
One other option which is very popular is the remotec 90. This is a $45 eight Button battery operated device where each button has three options: tap, double tap, and long hold. So a total of 24 options. This could make a nice auxiliary controller, particularly if you want something which isn’t glowing in the dark, an issue in many home theater set ups. 
And more home theatre ideas
You should also look at the project reports list for audio video projects: there are some cool home theater set ups there.