Most of the horde configuration files are in /etc/psa-horde/horde. For instance, to change "Welcome to Horde" to "Welcome to yourdomain.com" you would edit /etc/psa-horde/horde/registry.php by changing the line:
'name' => _("Horde"),
to:
'name' => _(preg_replace('/^webmail./', '', $GLOBALS['HTTP_SERVER_VARS']['SERVER_NAME']) ),
To change themes, edit line 292 of /etc/psa-horde/horde/prefs.php (I use simplex)
To enable spellcheck, edit /etc/psa-horde/horde/conf.php by adding this line to the end:
$conf['utils']['spellchecker'] = 'usr/bin/aspell';
I'm sure there are many other changes that can be made here. Hope this helps.