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Exactly.

But it's better to create new style which extends main style and have at least custom overall_header.html and overall_footer.html. It will to separate your custom code and easier update style.

With phpBB, I believe there is a way of sharing the authentication, authorization and probably other common tables and having separate tables for storing forum content. With earlier versions of phpBB, the configuration file used constants like a language file (like old versions of osCommerce). ...

... based on your description and restrictions, but I am guessing about bridging phpBB for your multisite concept. That is, installing three separate sets of phpBB tables with common auth tables or adding a parent category for each site with one set of tables and only linking to the applicable ...

... submission. ⋅ Feature: new 'Rating Table' question setting 'Results Options -> Frontend Results -> Expand Sub-Questions': displays separate results chart for each sub-question. Single chart with aggregated answers from all sub-questions is displayed if disabled (previous behavior). ...

You can have a separate category for Videos and separate for Articles. You can have 2 additional usergroups: 'Videos Publisher' and 'Articles Publisher'. Categories can be configured to allow submit for one of these specific usergroups ...

... HikaShop Multi-Vendor Membership Comp Emerald Subscriptions Internal Communication Community Builder Group Jive Kunena Forums UddeIM Project Fork Separate Comps Fabric Jevents Appointment Booking Pro Smart Slider3 Vertical Menu (Nextend)

It's possible to have separate Media Manager options 'Image Path' and 'File Path' for each site.

Yeah, we want the users to be registered on all sites, but not necessarily show up in search of maps or such. Sounds like we need a separate registration field for "active sites" which we check for when displaying search results, something like that?

Yes, it's possible: Joomla users and single jomsocial users table can be shared, other JomSocial tables can be separate per site as usual.