To change look and feel of Oracle Forms open the following file
Regarding different version of Oracle Forms 11g you can search about formsweb.cfg file in MiddleWare Home also.
Locate element lookandfeel in any section like the following
[sepwin] lookandfeel=Generic [webutil] lookAndFeel=windows
lookandfeel can be one of the following
Note change lookandfeel doesn't require restart service to take effect, It effects directly after saving formsweb.cfg file.