Open

templates/default/detail.html

Find

Code:
<div id="video_player">
<object width="425" height="355">
<param name="movie" value="http://www.youtube.com/v/{$id}&rel=1&border=0"></param>
<param name="wmode" value="transparent"></param>
<embed src="http://www.youtube.com/v/{$id}&rel=1&border=0" type="application/x-shockwave-flash" wmode="transparent" width="425" height="355">
</embed>
Replace With

Code:
<div id="video_player">
<object width="425" height="355">
<param name="movie" value="http://www.youtube.com/v/{$id}?fs=1&amp;hl=en_US"></param>
<param name="wmode" value="transparent"></param>
<param name="allowFullScreen" value="true"></param>
<embed src="http://www.youtube.com/v/{$id}?fs=1&amp;hl=en_US" type="application/x-shockwave-flash" allowfullscreen="true" width="425" height="335">
</embed>