Lately I have seen my images being more grainy than before although I have been using the excellent Sigma 18-35 lens on my Panasonic G7.
I noted I had changed image style from Standard to Scenery.
Does anybody know if the image style can affect quality of the image and not only colors? When I say quality in this case I refer to definition/sharpness.
I will do some tests on this but I'd like to hear other's experiences as well.

Scenery preset likely applies more sharpening. As a matter of course I always set my G series cameras to "NATURAL", I find the colors and tonal values more pleasing. Any changes needed are done in post.