Ok, so to start off I'm no Theme developer, but I have a little bit of experience with PHP, and quite a bit of experience (Several Years constant work) with wordpress in general.
I have a current client that wants to use a wordpress theme he saw exampled on another website. I found out that it's the (now outdated) Ridizain theme, which hasn't been updated in a few years. I know the theme is a modification of the Twenty Fourteen theme, so I could probably make some CSS Changes to 2014 and come up with the same look.
However, I'm wondering If I could just download the Ridizain theme and make any necessary changes to update it for the latest version of wordpress. I have a test server that I run and it does not seem to have any direct conflicts when I install it.
Is there a resource out there for Theme developers that details what needs to be changed/updated in a theme to bring it in line with each new version of wordpress? If not, where would I begin looking for what needs to be changed/watched out for?