How do I adjust the line thickness to be thinner?

Are you talking about the outline of the text?

If so, the line thickness is determined by the thickness of the laser and the effect of the burning on the material. The only real way to change line thickness is by changing the optical conditions of the burn and possibly the material to which you’re burning. You could potentially change lens or try different defocusing techniques to change the actual line thickness.

If you’re trying to get as thin a line as possible then focus on these main things:

1. get the finest focus you can achieve
2. potentially get a lower focal length lens
3. use the minimum amount of power necessary to make your mark
4. change materials