The Version 21 update included a brand new materials editor/library. Lit appearance is now known as Lit LED.
To find this:
1. Open 'Materials'
2. Locate the 'ArtiCAD Default Library'
3. Find the 'Misc' folder
4. Here you will have 'Lit LED'
To change the brightness of Lit LED, click 'Customise' and change the diffuse.