While the austere app was never unsightly, it certainly didn’t conform to any Android design guidelines. You can see in the new version there are on-screen back buttons at the top left and New Message and Search buttons on the top right. The updated app also uses a nicer font, more conducive to reading on higher-resolution screens.
The update should be rolling out to the Play Store in the coming days or weeks (WhatsApp has not publicized the update yet) but it’s available to download from the company’s website. If you’re already paid the 99c necessary for a lifetime subscription to the app, you won’t need to do so again when sideloading this version.