No. Google does not give extra credit for a custom design. Its guidance focuses on helpful, original content made for people, and on page experience. Page experience covers things like speed, working well on mobile and avoiding pop-ups that block the page.
Google recommends responsive design, which means one site that adapts to any screen size. Most modern templates and custom builds do this. What matters is how well it is done.
The bigger risk with templates is not the design itself. It is heavy code, too many add-ons and leftover demo text. Those can slow a site down or make pages less useful. A custom build can have the same problems if it is done badly, so test either way.
Before you buy any template, test its demo site. Run the demo through Google's free PageSpeed Insights tool and open it on your phone. If the empty demo is slow or awkward on a phone, it will not get better once you add your photos and content.