Docentric AX Designer is actually MS Word with a special Add-In whereas report templates (aka designs or layouts) are just ordinary Word documents. This also means that you can edit report templates (their static content such as colors, fonts, formatting, text, etc.) using plain MS Word.
Documentation for Docentric AX Designer >>
Docentric report templates used for MS Dynamics AX 2012 and Dynamics 365 for Finance and Operations are the same (100% compatible) as well as all designing techniques. The same goes for the rest of Docentric AX report artifacts such as DSP classes. That is why you can migrate all Docentric reports from AX 2012 to D365FO solution with almost no effort.