emergency battery and dimmer in parallel
As drawn this is an incomplete idea, but it looks like you're trying to use a UPS to drive your main lighting all the time.

If this is to meet a Swiss / EU saftey requirement, consider purchasing commercial or industrial-grade emergency lighting. This is not a good first project for a hobbyist, because mains power is hazardous and quite unforgiving. Since you indicate you work with theater lighting I assume you're a licenced master electrician.

Assuming the battery is 12V sealed lead-acid (SLA), there must be a float charging circuit to keep the battery fully charged. There must also be a contactor (relay) to switch on the emergency lights when mains is unpowered. This can be accompished with a form B relay using mains to power the coil, and using the normally-open contacts to close the emergency lighting circuit when mains is in fault.

When selecting the battery, be sure to consider the actual electrical load of the lights. If the lighting draws 30 Amps and the target runtime is 20 minutes, then the battery capacity needs to be 10 Amp-hours. This is a big, heavy battery, which will require regular maintenance. I don't think this will be practical. In a mains fault condition, the design goal is to light the exits, not light the whole stage. This is why most commercial emergency lighting systems use their own independent low-voltage lights. This reduces the load and eliminates the need for a costly 12VDC to 240VAC inverter.

All commercial emergency lighting systems I've seen in USA have their own self-contained lamp that illuminates when mains is unpowered. But if the main 240VAC lights must be powered in an emergency, then the system must include a 12VDC to AC inverter to power the lights. Preventing this inverter from back-powering the mains is important, because otherwise you risk the saftey of the lineman who is trying to restore the mains power. You can't just start powering the mains grid with your own battery powered inverter. This is the type of problem faced in any UPS design.