[_ck_]

Just install my bbpress theme switcher plugin, disable the dropdown.

Then replace function bb_ts_get_theme() {

with something like this (untested, will need some editing)

function bb_ts_get_theme() {

$theme="kakumei";  // default fallback

$domain=strtolower($_SERVER['HTTP_HOST']);

if (strpos($domain,"first-domain.com")!==false) {$theme="1st-theme";}

if (strpos($domain,"second-domain.com")!==false) {$theme="2nd-theme";}

return $theme;

}