Portfolio – Strange right border on individual entries

I’ve made some custom modifications to a portfolio grid so that it looks like this:

As you can see, each grid entry has a 1px right border. It is not in the image, and it doesn’t appear on the iPad – only on desktop browsers. (The screen shot was taken in Firefox 24.0.)

Here’s the code for the actual page:

<div width=”90%” style=”background:#ffffff;margin:-20px 25px 0px 25px;padding:0px 0px 10px 10px;”>
[av_portfolio columns='4' items='-1' contents='no' preview_mode='auto' image_size='portfolio' linking='' sort='no' paginate='no']
</div>

And here is the custom CSS for the portfolio:

#top .grid-entry .inner-entry {
margin-right: 10px;
box-shadow: none !important;
}

#top .flex_column {
margin-bottom: 0px;
margin-right: 10px;
}

.responsive #top .flex_column {
margin-bottom: 0px !important;
}

Any thoughts? Thanks!