If you want to make dashboard become invisible, you can change opacity value, 0 is invisible, 50 is half visible, 100 is 100% visible / transparency. You can copy following code and paste onto your plurk layout css editor :

For dashboard 100% transparency (totally cannot see) :
div#dashboard_holder {filter:alpha(opacity=0) !important;opacity:0 !important;}
div#dashboard_holder:hover {filter:alpha(opacity=100) !important;opacity:1 !important;}

For dashboard 30% transparency :
div#dashboard_holder {filter:alpha(opacity=30) !important;opacity:0.3 !important;}
div#dashboard_holder:hover {filter:alpha(opacity=100) !important;opacity:1 !important;}
Picture below shows 30% transparency of my plurk dashboard :


For dashboard 50% transparency :
div#dashboard_holder {filter:alpha(opacity=50) !important;opacity:0.5 !important;}
div#dashboard_holder:hover {filter:alpha(opacity=100) !important;opacity:1 !important;}
Picture below shows the 50% transparency of my plurk dashboard :


When you are using your mouse to point on your dashboard, it will change your dashboard into 0% transparency just the same as the picture below :


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