setting background color

This is a FAQ, but my searching about did not help. I want to make
background color permanently black. I'm using the Twice style. 
In startup I put the line:

     fbsetroot -solid black

This works, but I get an error:

     fbsetbg: Can't find wallpaper style

In fluxbox manual it says I can prevent this with the command

     session.screen0.rootCommand: fbsetbg -f   \
     ~/backgrounds/zimdib_dark.png

I don't understand. The -f option selects the background, but I want to
ignore it. Also, I have no such ~/backgrounds folder. 

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