I'm not aware of a way to change the reticle brightness. I know you can change the reticle shape and color, but not brightness. If I'm wrong, I would like to know how to change it too.
I spoke with the customer service guys about this and they said the brightness of the reticle is fixed and the only way to brighten it is to adjust the screen brightness. I use the bold cross hairs on the opposite color and have not had any issue. If I'm on white hot my reticle is black if on black hot it's white. Hope this helps.
Adjusting the rear focus lens will sharpen the cross hairs I thought the exact same thing when I got mine 42mm 640.
The rear focus lens and contrast. Also remember it depends on color palette vs reticle color also