Ok, i did some testing and i got it. Here you go, elon:
First, rename your "on" texture to +0~SCREEN_EY and the "off" texture to +A~SCREEN_EY. Then add these in a new or existing wad file.
Then openup your lights.rad/valve.rad file and add this line:
+0~SCREEN_EY 230 85 0 1200
Now go into Hammer and make a brush with your "on" texture on it. Turn this brush into a func_wall and name it screen_light. Then go the Style option and select Grouped. If you don't see the Style option, turn off smart-edit and add a new key and value:
key: style
value: -3
Next, place a light entity anywhere near and name it the same as your brush, in this case: screen_light. Set its color and brightness to 255 255 255 0.01, so that it won't emit any light of its own.
Now make a func_button and let it target the brush (screen_light). Goto its flags and check Don't move and Toggled.
Thats it, save and compile this map with the ZHLT 3.0 or higher. 3.2 is out, so use that one. Don't use older tools.
If you use the button ingame, the screen texture goes to its off version and the orange light dissapears. If you use button again, the screen texture goes back to its on version and the orange light reappears.
If you can't figure this out, i got an example map and everything ready for you.