You won't be editing the raw html exactly, but how to do this depends on how the code was inserted in the first place.
1) Go to the "Edit" menu and select "Edit Header..."
2) If you see the following code in the window that opens, just delete the code and click OK. That should take care of it.
<BODY
BGCOLOR="#000000" TEXT="#ffffff" LINK="#ffffff" VLINK="#ffffff" ALINK="#FF0000" >
If you don't see the above code in the window that opens, then that means the HTML Tool was used to enter the code.
1) This means you have to look for a box on the page which will probably appear to be empty.
2) Select the box so it's outlined in blue.
3) In the Object Properties window click on "Edit HTML". This opens the Insert HTML window.
4) Click on the "Header HTML" tab in the Insert HTML window.
5) If the above code isn't there, then you have selected the wrong box. Click "Cancel" and keep looking for the right box. If the code is there, then click "Cancel" and, with the box still selected, click the delete button.