Gray borders appearing at roGridScreen while on HD

That's nothing to do with the SD screen. It's because there's a viewport cut into the graphics plane for the grid itself. At the edge of that viewport, you'll sometimes get a line like you're seeing (I think it depends on the color you choose) where the grid element, which is on the video plane, transitions to the graphics plane. It falls outside of the safe zones, though, so it's likely to be cut off by the overscan on most televisions. You'll notice, as you scroll left and right that line is also where the posters get clipped.

Thanks for the reply! You are right, double checking it, in my TV it doesn't appear on 720p, just 1080p.
I tried several grayscales on the background and for the line to dissapear (or get close to unnoticeable) it has to be pretty close to
#000.

Will be great if someone from the staff chime in and comment on the status of this issue. It will look as a bug in some TVs.

I get a horizontal line right around the 138 pixel mark (the limit for the overhang) on 1080p but not 720p, when the overhang and the bkgrnd were the same color. As you say the solution is to make the bkrnd color black, which i didn't consider much of a solution. The fact that you can't theme the text in the one configuration where text can appear under the thumbs tells me Roku would like you to have a very dark background on your grid.