That’s what I would do. I would use a border on the left and right of each button, then use a background image for the background of each button. This would mean that the separators would not be gradiated (<–what’s gradiated mean?), but it would be hard to notice.
Assuming your code looks something like this:
The CSS would look like this (in addition to all your current declarations):
You could even change the colors on the hover state if you’re inclined.