Re: the grey boxes – I had a similar problem after upgrading from 2.6 to 2.7.
It looks like the “Previous”and “Next ” areas – which were defined as “<div>” objects in previous versions – are now defined as “<button>” objects.
This means that they’ll inherit any button css properties defined in your theme.
My theme is based on “underscores”, which defines buttons as having (amongst other properties) a grey background – hence the boxes.
I’ve over-ridden this by putting the following in my theme’s css file, which loads after the underscores file (and after LB+CB, although that’s probably not relevant):
#cboxPrevious, #cboxNext {
background:transparent;
border:0 none;
box-shadow:none;
text-shadow:none;
}
However, I have another problem with an outline of the button (in blue) remaining after I click a link (even after I move to the next image). I suspect that this is similar in nature, but haven’t tracked it down yet.