Eventhought I had copy all the php files from default-templates into my current theme folder. The appearance will turn weird if I press any of these buttons ie.Question, Unanswered, Ask a question , Search. However there will be no problem if enter into Q&A section via Q&A widget.
For your information I did not add the following two lines
add_theme_support( 'qa_style' );
add_theme_support( 'qa_script' );
to my theme's functions.php file because it will made the appearance weird regardless of from where you enter into Q&A section.