Making all posts have a gradient would lag people more than it does now @___@. Plus, I think having a gradient would make things too complicated and the forum would look like one big designing mess. Having it simple for the posts make it much easier to read. I think it looks fine the way it is o__o.
Well, I mean a small (like 5KB .gif small) gradient would work best, and it wouldn't lag people.
If not the gradient, please change the black borders to the colors I've suggested, please (at least for the postbit). With that, the layout will be a lot smoother and a lot more sexy.
Here's an example of what I mean:
It fits the theme better, methinks.