Theme Simple has been updated to version 4. It applies to derivated themes Simple Grey, Simple Black, Simple White and Simple Sunset. The main changes are on the page of the photo as you can see on the 2 following screenshots, comparing version 3 and version 4.
As you can see, changes are not big, visually speaking. Actually changes are huge, technically speaking. We use the same HTML templates as in default theme Elegant, and it makes the theme much simpler to maintain and we avoid a few bugs that were complicated to fix until now (such as distorted photo on very small screens).
If you encounter any specific customization issue with this new version of theme Simple, please contact Piwigo.com support.