Current stable version of Opera is 12.0.2, let me tell next release is Opera 12.1 but not Opera 12.50, today Opera has released a Beta release candidate for  Opera 12.1 which close to Beta stage adds Retina support for Macs with with plenty of bug fixes and big core grade, one significant core upgrade is WebM improvement.

“As an extra bonus for our Mac users running the latest MacBook Pros, Opera 12.10 sees the addition of "Retina" support. With twice the pixel density, this should give much crisper text and detail to the Web experience. The build comes with a updated skin, full of pretty, high-resolution icons. “Opera desktop team blog reads.

Opera 12.1

You can install this release over Opera 12.50 next on your Computer but you won’t get automatic updates to Opera 12.1.You can find the changelog and download Opera 12.1 beta RC from Opera desktop team blog.