func_lightstyle
Creates a custom lightstyle to be used by other lights

All you have to know about the func_lightstyle entity:

Make an entity with these fields:
classname func_lightstyle
style x
message s

Set style to a number between 12 and 31.  Above that and you'll interfere with triggerable lights, so if those fill up, use 62 and start moving down (62, 61, 60, etc).
Set message to a string of lower case letters (such as "abcdefg").  You can use numbers and other special characters (but not ") to get some very interesting effects.

There appears to be a 54 character limit in the light's message string.  More than that, and you'll get the effect of having numbers or special characters instead of what you wanted, and I don't know how to control which ones you get.  This is unfortunate, as 54 characters is way limited!

Give it a targetname to make it triggerable.  The Start Off spawnflag (spawnflags 1) works like in lights.  There is also an Always Turn On spawnflag (spawnflags 2), which means the lightstyle will turn on every time it's triggered.  In this way, you can have multiple lightstyle entities controlling the same style number, and turn on different ones when needed (which will override the previous setting of the style and affect all lights using it).

You can, technically, give the lightstyle an origin and a light value, causing it to emit its own light and use its own style.  This is fine, but if you need more lights to use that style, use normal lights instead of creating another func_lightstyle.