Before I tell you the answer I have to say this is the ugliest hack I have ever done...and I hate frames to begin with .index.html
<html><head><style>html,body{background:GRAY;margin:5px;}</style></head><body><iframe src="frameset.html" style="width:100%;height:100%" frameborder="0"></iframe></body></html>
frameset.html
<html><head><style>frame{border-bottom:1px solid GRAY}</style></head> <frameset rows="65%,5%,30%"><frame src="http://www.google.co.uk/" frameborder="0"><frame src="http://www.google.co.uk/" frameborder="0"><frame src="http://www.google.co.uk/" frameborder="0"> </frameset></html>
There you go. You can play with the margins and borders until they are the thickness you want.