I didn't have thin eyebrows by the width, but I did have very sparse eyebrows. There just wasn't much hair there, but they have gotten a bit darker since I started T. They're still thin but I can definitely tell that overall they're darker. It's one of those things that subtly gives people cues, so them darkening even just a little has helped a lot with passing. I wouldn't worry too much about it, and if it's bothering you in the mean time, follow everyone's suggestions and use a little bit of makeup. Eyeshadow or eyeliner (used sparingly!) can darken them nicely, and it's completely unnoticeable.