Post grid custom

Hello,

I made a customization on post grid to have a short space between thumbnails
whit this css:

div .av_one_third {
margin-left: 2%;
width: 32%;
}
div .av_one_fourth {
margin-left: 2%;
width: 23.5%;
}
div .av_one_fifth {
margin-left: 2%;
width: 18.4%;
}

This css seems to work,
but I get a problem when I resize the browser and thumbs are aligned on one or two columns.
This is the link: http://www.birikkina.com/wordpress
Try to resize the browser window as small as you can, and you will see they the thumbs don’t fill the grey box under each image…

Is it possible to fix this issue?

Many thanks,
Pietro