Reduced thumbnail size due to increased window picker spacing/padding

The theme update changed the window picker spacing and padding from 16px and 16px to 48px and 64px respectively. This reduces the size of the window thumbnails in a very noticeable way and makes it harder to differentiate windows. The new spacing/padding is even bigger than it was before 64bbad1b (32px/32px) which reduced both to 16px according to this design document with the intention of maximizing the thumbnail size.