The easy answer: no. But there's more to say:
Not without removing the margins from all of the buttons in your toolbar, and certainly not from the Robo skin editor. In webhelp, your buttons are set in a table and the function writeToolBar() (in whtbar.js) creates that table. I won't recommend changing that, because it has a lot of dependencies and this script is also used to write the minibar (where the browse sequence arrows are located).
If you want to change some styles, take a look at the function writeStyle(bMiniBar), but everything you change will affect all the buttons. Manually adding the html-code to the string might work, but because the function writetoolbar is used for two toolbars, you need to make sure the html only ends up in one toolbar. I took a peek, but I didn't see anything obvious to do this.
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