How should I develop a WordPress theme being non-PHP (or just a frontend developer) developer?

If someone being a frontend developer (not fullstack) wants to start selling theme online then what standard should he follows. Also the developer has only basic knowledge of PHP and he's not core PHP developer. His skillsets are HTML5, CSS3, JavaScript, jQuery and so... Still he can google out things to achieve his goals for WP and implement.

He is looking for a standard and valid guideline to develop multi-purpose theme considering future updates too. Means code written once shouldn't be changed per the future updates by WP.

Thanks in advance :)