I think they are using a custom one made by their designers. You could ask a designer to try to make something simmilar, but with customization for you.
In regard to the theme, it's really nothing special...if anything, I've never liked the design of Mashable.com (I'm a daily visitor)
My FireFox goes funny when I visit the site. And I think it's Wordpress with custom theme, it feels like a modification of a default theme though.
BTW If u want to know.. On which platform a site is built on.. You can try www[dot]builtwith[dot]com Comes in handy. Hey what theme u are using ?? Its really looking like a Mashable clone.. KUDOS !