Trimming or shaving body hair makes the body hair appear to be thicker but it actually is not.
When you trim or shave your body hair the shaving or trimming of the body hair gives the body hair a blunt tip which makes it appear thicker.
The body hair grows the same thickness whether you shave or trim it or do nothing to it.
As well as appearing darker the body hair can also appear thicker but in reality it is no different from when you didn't shave it.
There's no way to actually make your body hair grow thicker or thinner as it grows out of your body and your body is the only thing that can change the thickness or thinning of the body hair if it were able to.