Rounded corners transparent .PNG

[Richard_RB]

How can I Remove the WHITE backgrounds from ALL left and right blocks?

I can't find the correct CSS.
Take the Categories block for example, if I add a rounded corner .PNG image with transparency to the top of the categories, instead of being transparent in the corners, the corners turn white. If I was using a white BG this would be fine but I'm using a dark gradient background so I also can't just use the same color. The only way for rounded corners to work for me is if I have image transparency.

The only reason I can think why the corners remain white is they must be set this way in the CSS but I can't find it. Please help?
Also, is there one CSS setting for all the blocks or one for each of them?

[Richard_RB]

It's okay I've found the correct CSS, not sure anyone would have been able to help me because it wasn't in the Global.css file. It was in my theme's style.css file.

#left_column div.block,
#right_column div.block {
background:url(../img/boxTop.png) no-repeat 0 0 #fff;
padding:0
}

As you can see, I just had to remove the #fff and my rounded corners no longer had white behind them. However, this also removes the white background from the actual catagories content too so I'm either going to have to put the white background back and adjust the padding to the rounded corner image moves above the white background or I'll just add the white background to the catagory list instead of the block itself.