If you ever want to duplicate an existing phpBB3 theme, you might be sorely frustrated when you can’t seem to get any of your changes to take hold. It is sometimes necessary to go to the ACP, and hit purge the cache, however there is a further trick.
First, duplicate the theme folder you want to start with, and rename to your theme name.
Second, you need to go to the theme folder, and open style.cfg, /imageset/imageset.cfg, /template/template.cfg, /theme/theme.cfg, in a text editor. All of these files contain name = value. Rename all of these to your theme name.
Upload all of these changes, then head to the ACP and go to the “styles” tab. Click install on your theme. On the left hand menu, go to each of the style components, and select install for each one. Then head back to the main styles page and click “details” for your theme. Select the imageset, template and theme from the dropdown menu.
Now you should be done!