If you’ve written a child themeChild theme A Child Theme is a customized theme based upon a Parent Theme. It’s considered best practice to create a child theme if you want to modify the CSS of your theme. https://developer.wordpress.org/themes/advanced-topics/child-themes/ for Twenty Fifteen, please note that some of the new pagination functions have been renamed for a bit of clarity:
the_pagination()is nowthe_posts_pagination()get_the_pagination()is nowget_the_posts_pagination()- These two functions now emit a “posts-navigation” HTMLHTML HyperText Markup Language. The semantic scripting language primarily used for outputting content in web browsers. class, instead of “paging-navigation”
@obenland‘s original post on new template tags in WordPress 4.1 has been updated with these changes.