Could you post a link to a page with the problem?

I don't think you have added the CSS to the right css file (I don't have to login to see this, you can remove the login)

The one you have to edit is: /Portals/0/Skins/MLTrendy-Brown_Cavinato/skin.css

Change this

body{
border-left:0px 0px 0px 0px;
margin:0px;
padding-top: 0px;
padding-right: 0px;
padding-bottom: 0px;
padding-left: 0px;
background-color: #333333;
}

to:

#body{
border-left:0px 0px 0px 0px;
margin:0px;
padding-top: 0px;
padding-right: 0px;
padding-bottom: 0px;
padding-left: 0px;
background-color: #333333;
}

and it should work...

The provider tries to simplify skinning of your image and link galleries. Those galleries are template based and their templates can be found here:

Providers\HtmlEditorProviders\Fck\FCKTemplates

You will find that each template can be build based on selected language, so you can build a complete different gallery for english while having a special look for Spanish

The main part of each template is the Master.Template.htm

you will see that the template references a .css file and is that file that you need to change to make the corresponding gallery to look as you want.

As a suggestion, you can try creating a new gallery template just by copying a template folder into a new folder and changing its contents. This will prevent you template to be overrited when a new provider version is installed.