The original bug is:
Log in to ACP is working, but if you click: "Return to ACP",(!!! in non default style !!!) it returns you, to board index, not ACP index...

After 30 mins, I soolved this bug.
Description of fix: the meta refresh url is: ../index.php; BUT the client is in the ./adm/ dir already...

Why "../index.php"? $phpbb_root_path variable is now: "../", 'cos, the client is now in ./adm/ dir, but the root isn't "./adm/", the root is "../";
so I removed the variable $phpbb_root_path; and now it works with ALL styles...

Tested: prosilver, "blank style".

Please commit my patch! thanks