。
The fog is getting thicker, isn't it?
Literal
Fog [subject-が] thick become-come-[progressive-ています] [confirmation-ね].
濃くなってきている is an aspect stack: 濃くなる (become thick) + ~てくる (process unfolding) + ~ている (ongoing state). The whole cluster means 'is in the state of progressively becoming thicker.' ね seeks agreement. Weather observation between two people.