Forum themes are using vendored copy instead of inheritance

[kkremitzki]

The FreeCAD and FreeCAD-Dark forum themes use a vendored copy of an upstream phpBB theme from an old version; the current version of phpBB has diverged from this copy, resulting in display issues. Instead of using a copy, these themes should use inheritance from the base theme, with the changes specific to our variant listed out directly, rather than existing as direct modifications to the theme's files. This is the proper fix to existing issues, and will result in more resilience to further upstream changes in future phpBB versions.

Reference: https://www.phpbb.com/styles/create/