Sub theming Seven theme
Creating a sub-theme is pretty basic:


Create a sub-dir for your themes in your configured sites dir (e.g. sites/default/all/my_subtheme).
Copy over the .info file of the theme you want to subtheme (e.g. themes/seven/seven.info) to your new theme directory with it renamed to match your directory (e.g. sites/default/all/my_subtheme/my_subtheme.info).
Edit your sub theme .info so that it also contains the line base theme = seven. You can optionally give it another name or description.


From there you should be able to enabled it and start using it. You can copy over other resources that you'd like override in your subtheme. In your example, you can create templates subdirectory (e.g. sites/default/all/my_subtheme/templates) and copy the simplenews templates that you wish to override.