The grid is hardly visible in all outdoor images, except those with very dark backgrounds or shadow areas.
How do I increase the brightness of the grid?
I don't think you can change the overlay color or brightness, but if you go into the crop tool you can hit "o" on the keyboard to cycle through different crop overlays which might be easier for you to see.
There isn't a way but you can...take note of your exposure slider setting, crank it down a couple of stops, tweak your crop, return your exposure setting to where it was.
Not elegant but I use it outdoors on my laptop when it is too bright to see what is going on.