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I never did find a way  to  make the header transparent, but I did come up  with a workaround. 

I wrapped the entire contents of the header.tpl  file in a div,  and used the same image as I used for the main background, and then I made both of them  static.  It's still not transparent,  but for all practical  purposes the visual effect is the same. 

In case anyone needs this is the future:
 

...in custom.css

#wrapper {
 background-image:url(whatever_image_you_got.jpg);
   background-repeat:no-repeat;
   background-position:right top;
   background-size:cover;
   background-attachment:fixed
}

...and then in a the div that surrounds everything in  header.tpl 
 

<div style="background-image:url(same_image_as_you used_the_first_time.jpg);
                 background-repeat:no-repeat;
                 background-position:right top;
                 background-size:cover;
                 background-attachment:fixed">  


</div>   

...this solved the problem,  and I probably would have thought of it sooner,  but I was I too headstrong on making everything transparent as opposed to actually achieving the visual effect. 

This was much  simpler,  and a whole lot less headache. 

I never did find a way  to  make the header transparent, but I did come up  with a workaround. 

I wrapped the entire contents of the header.tpl  file in a div,  and used the same image as I used for the main background, and then I made both of them  static.  It's still not transparent,  but for all practical  purposes the visual effect is the same. 

In case anyone needs this is the future:
 

...in custom.css

#wrapper {
 background-image:url(whatever_image_you_got.jpg);
   background-repeat:no-repeat;
   background-position:right top;
   background-size:cover;
   background-attachment:fixed
}

...and then in a the div that surrounds everything in  header.tpl 
 

<div style="background-image:url(same_image_as_you used_the_first_time.jpg);
                 background-repeat:no-repeat;
                 background-position:right top;
                 background-size:cover;
                 background-attachment:fixed">  


</div>   

...this solved the problem,  and I probably would have thought of it sooner,  but I was I too headstrong on making everything transparent as opposed to actually achieving the visual effect. 

This was much  simpler,  and a whole lot less headache.