There is a script file that makes all SharePoint views in the lists look modern:
/_layouts/15/clienttemplates.js
You can add thea link theto this file toin the list's web part:
If you don't include this link it will look like so:
Also, you might not need it, but if you are updating Visual Studio sandbox solutions for Office 365 from SP2010 to SP2013 you need in all schema.xml files that contain list definitions include a JSLink property: