gUI3 isn't broken, so you can't fix it. It was made for MoP, and works nicely with MoP. But MoP is over and done. We're living in a WoD world now. And you'll never get that exact look with any other addon, since gUI3 - and everything else I make for that matter - uses custom made textures and artwork that are NOT public domain, meaning other addons can't include them without my written permission.
However, if the cubic, narrow, blocky, and in my own humble opinion outdated look of gUI3 is something that still appeals to you, I recommend keeping an eye out on
www.friendlydruid.com the upcoming weeks. There might be an announcement of interest there in a few weeks.
Oh, and if anybody does update gUI3 for WoD, I do in no way give them permission to publish it. I might come across as a dick here, but gUI is my project, and I'd prefer it to stay that way.