Can I use a 18AWG wire to connect a low current (LED) light to the ceiling?
I just installed 7 lights of an identical type.  Here are my thoughts:
First 18ga wire is adequate. !8 AWG is normally considered adequate for 10 amps in lighting circuits. You should not bury that wire under anything, as it is designed for free air use.  But for one fixture, drawing less than an Amp, it will not get warm.
Second, the connector on the light is firm, but designed for 14ga solid copper wire.  Stranded wires may not have the requisite stiffness to properly insert into the connector, and there is the risk you could have a &quot;hair&quot; of wire separating from the strand, which could cause a problem (short).
They do not have a provision for ground wires, and since everything is plastic except the mounting bracket, I simply tied my ground to the metal mounting bracket.
Note for anyone else, to mount these lights in a box, I found it necessary to drill an additional hole in the mounting brackets.  They appear to be designed for non-US markets, and while they will work fine, some things, such as hole spacing, are not ideal for mounting on common US boxes.
How are you planning on mounting this, and running the wire to it?