How to delete the media container border, in the popups for FlexViewer 3.0?

I'm trying to delete the image rectangle box in my popups, so there are no gaps around my image.

Here what I did in Flash Builder:
1- In the PopUpMediaBrowserSkin.mxml I deleted the code:

<s:Rect id="borderRect"
left="0" right="0" top="0" bottom="0">
<s:stroke>
<s:SolidColorStroke id = "borderRectSymbol"
color="black"
weight="1"
<s:stroke>
</s:Rect>

2- open the default.css file (default package) and included the following namespace,
@namespace skinComps "com.esri.ags.skins.supportClasses.*";

Also inlcuded the following skinClass reference for "PopUpMediaBrowser",

skinComps|PopUpMediaBrowser
{
skinClass: ClassReference("com.esri.customSkins.PopUpMediaBrowserSkin");
}

Any suggestions, please? I new in Flash Builder