Single post in A Somewhat Extensive Guide to BBCodes

Gradients aren't done with BBCode. It's all CSS and style codes for that. You can actually google CSS gradients and find tools to help you put the background together for you. (it's honetly just plugging in a lot of numbers anyhow lol) Keep in mind though, you'll have a lot more styling to do since hide boxes, tables and words have to remain legible across the whole gradient (easiest way to do this would be to use a text box that's a solid color.)