I had posted about this earlier today (it hasn't been approved yet, but if the mods are watching you can delete it as this thread answered my question). I noticed the pixels too although less on the light theme. I just got the 635 from Amazon for 62 dollars a Tmo device, I have a Nexus 5 on Tmo;s $30 plan, so it was easy to switch SIMs. I bought it as I need to learn Win Phone for work (I am IT support), also I am interested in it, I want to learn it.
I was going to return this back to Amazon (who no longer sells it so I would be unable to get an exchange), but this tells me it is normal, and not defective. The white background setting helped, Considering this is an LTE phone (I live in NYC so LTE is not an issue except my apt where my N5 can't even get LTE), for $60+ I would hate t return it,, I do not now think it is defective. Maybe coming from a higher end phone like the Nexus 5 and for $400 I would hope it was a higher end screen, I need to get used to the screen. I am not abandoning Android, but I am impressed with Win Phone, this will be my backup OS, I am not a fan of iOS.
Sorry about the Android avatar, I am coming from androidcentral.