I was recently asked via support how to customise the v14.2 Debian FHS, that isn't quite the case. The Debian package install puts things in slightly different places.

Whilst the file paths don't match the upstream docs, the phpBB v3.0 manual will likely still be useful. Obviously you can ignore the install section and the upgrade section will also be irrelevant. However, the administration section [Chapter 3. Administration Guide; starts on page 20] should still be totally relevant.

You will also find some local documentation on your phpBB server regarding the phpBB Debian package (explicitly called phpbb3) and phpBB itself within the /usr/share/doc/phpbb3/ & /usr/share/phpbb3/docs/ directories. Some of that will relate directly to the Debian packaging and will be irrelevant to you, but it's worth noting anyway.

The phpBB configuration files will be within the /etc/phpbb3 directory. The full code of the 2 themes that it ships with (prosilver & subsilver2) can be found in /usr/share/phpbb3/styles/. The phpBB website code itself can be found within the /usr/share/phpbb3/www/ directory. The remaining phpBB assets can be found in /var/lib/phpbb3/.

A word of warning though, directories within the /usr/share tree are managed by the Debian package manager. So any changes that you make within these directories (e.g. within /usr/share/phpbb3/) may be overwritten by future updates. So you are advised to not make any changes there. It is recommended that phpBB files that you manage (and don't want overwritten) should be stored in /var/lib/phpbb3/ or /etc/phpbb/. These files will NOT be overwritten by future updates and are also included by default in TKLBAM (our custom backup tool) backups. Note that by default, no files from /usr/share are included in backups (another reason not to edit those files).

I have written a little custom script to copy and edit the default theme (prosilver). You can see the script on GitHub. To download and run it, follow these steps:

git clone https://gist.github.com/456cd69fbf7e4d7c92a41f8fad2fc601.git bin
bin/new-phpbb-theme.sh

When you run it (the last line) it should return a result like this:

Your new custom style is named custom_style, and can be found in /etc/phpbb3/styles/custom_style

Once you've done that, you'll need to log into phpBB via the web interface. Once you have logged into the moderator interface as admin, then log into the ACP (Admin Control Panel) via the link towards the bottom of the page (you'll need to enter your password again). Once logged into the ACP, then click on the "Styles" tab. You should see "custom_style" in the "Uninstalled styles" section. Please click the "install" link to the right of that. You will probably wish to make this the default style by selecting the appropriate radio button. Once you have done that, then I also recommend that you "deactivate" the prosilver style (so users can only use your custom style).

You are now free to mess around with your new custom style which you can find in /etc/phpbb3/styles/custom_style.

Good luck and hopefully that helps.