Why reinvent the wheel? It already exists in thousands of forms, many of which are open source. WordPress, for example, was a fork of another abandoned open source platform and it now powers over 26% of the web. But if you must undertake this odd adventure, make sure it's different with a purpose and solves problems others aren't.

Agreed with Nicky. A few other considerations worth mentioning. Using a theme will require more effort to customize layouts/looks to your exact specifications, so you have to ask yourself if ease-of-deployment and a perfected look (that isn't tested yet) worth the risk of more difficulty in layout customization. And if you plan on doing design/layout AB tests or make regular changes to look/feel, how easy will it be to do those things? You might be able to get the best of both worlds using a powerful and/or clean theme and layer on something like Beaver Builder to be able to iterate much faster and streamline your design workflow tremendously. That being said, if the website will be a set-it-and-forget-it project I'd likely recommend custom from the ground up. Otherwise, for small business, consider working on a minimum viable deployment with a theme you can dig into deeply and layout builders that you can quickly iterate on.