Actually - it only fixed it temporarily.... until the next time I opened calibre. Then it went back to the heavy lines. I didn't like the new font anyway so I decided to switch back to Tahoma and the stars were back. Yay!!
BUT... I just opened calibre again and its back to heavy lines.
It made sense it might be the font (although I had never changed the font). But this is quite odd. The only changes I ever make to calibre are to update it every now and then.
So if anyone else had any suggestions I'm all ears.