Oh, and you can use any icon you want as a bookmark, at least in recent builds, the catch is, the icon must be found in the default theme folder, not current theme you are using.
For instance I made a little white icon with transparent background (it must be white for the filter to color it proper). I called mine bracket.png for this experiment, and then referenced it like this.
"curly_icon" : "bracket",
And there you have it.
I noticed that the cross icon no longer worked, and then I noticed the cross icon is gone in the default theme. I tried dropping a test icon into my current theme, but it never would get picked up, so I tried the default and it worked. So really you can use anything as long as it exists in the default theme.