as you suspect, its no walk in the park…  a bit difficult actually..  first,  your two sites would have to share the same users database..  possible if they use share the users db tables in wp..  they would have to be on the same physical server…  and not likely how you have it set up now.. 

another option would be to use the multisite portion of wp…  once install of wp and as many ‘network’ sites as you want…  each site is unique and independent, but they can share users…  and you could put the forum at the main site and have all network sites redirect to the main forum site..   but again probably not what you are doing for the different sites now…  so would require some work to get there…

as to the css, that is more of a problem…  since each network site is its own, they would of course have their own css… the main site would have its own too…  the wp api is such that you could know where they come from and load the appropriate wp theme on the main site… but that is custom work – might even be a multisite plugin, not sure…