This is what I'm using for our site, just change the hex color's to what fits for you and place inside the CSS section.  Hope this helps some.
 
.SelectedTab {
BACKGROUND-COLOR: #383838;
COLOR: #FFFFFF;
BORDER-WIDTH: 1px 1px 1px 1px;
BORDER-STYLE: solid;
FONT-SIZE: 10pt;
FONT-FAMILY: Verdana;
BORDER-COLOR: #666699;
TEXT-ALIGN: CENTER;
PADDING: 2px 2px 2px 2px;
SPACING: 0px 0px 0px 0px;
}
FORM
{
BORDER-RIGHT: medium none;
PADDING-RIGHT: 0px;
BORDER-TOP: medium none;
PADDING-LEFT: 0px;
FONT-SIZE: 10pt;
FONT-FAMILY: Verdana;
PADDING-BOTTOM: 0px;
MARGIN: 0px;
BORDER-LEFT: medium none;
PADDING-TOP: 0px;
BORDER-BOTTOM: medium none;
FONT-FAMILY: Verdana
}
.UnSelectedTab {
BACKGROUND-COLOR: #000000;
COLOR: #FFFFFF;
BORDER-WIDTH: 1px 1px 1px 1px;
BORDER-STYLE: solid;
FONT-SIZE: 10pt;
FONT-FAMILY: Verdana;
TEXT-ALIGN: CENTER;
BORDER-COLOR: #666699;
PADDING: 2px 2px 2px 2px;
SPACING: 0px 0px 0px 0px;
}
.AltGridRow {
BACKGROUND-COLOR: #000000;
COLOR: #FFFFFF;
FONT-SIZE: 10pt;
FONT-FAMILY: Verdana;
 BACKGROUND-IMAGE: url('');
}
.NormGridRow {
BACKGROUND-COLOR: #000000;
COLOR: #FFFFFF;
FONT-SIZE: 10pt;
FONT-FAMILY: Verdana;
 BACKGROUND-IMAGE: url('');
}
.ContentBox {
BACKGROUND-COLOR: #000000;
BORDER-WIDTH: 1px 1px 1px 1px;
BORDER-STYLE: solid;
PADDING-TOP: 0px;
MARGIN-TOP: 0px;
BORDER-COLOR: #666699;
 BACKGROUND-IMAGE: url('');
}
.ForumCategoryHeader {
BACKGROUND-COLOR: #000000;
BORDER-WIDTH: 1px 1px 1px 1px;
BORDER-STYLE: solid;
BORDER-COLOR: #666699;
PADDING-LEFT: 5px;
COLOR: #FFFFFF;
PADDING-RIGHT: 5px;
PADDING-TOP: 5px;
PADDING-BOTTOM: 5px;
FONT-SIZE: 10pt;
FONT-FAMILY: Verdana;
}
.ContentBoxTitle {
 PADDING-RIGHT: 5px;
 PADDING-LEFT: 5px;
 MARGIN-TOP: 0px;
 PADDING-TOP: 0px;
 BORDER-TOP: 0px;
 BACKGROUND-COLOR: #000000;
 COLOR: #FFFFFF;
 FONT-SIZE: 10pt;
 FONT-FAMILY: Verdana;
 BACKGROUND-IMAGE: url('');}
.ContentBoxTitleNoBackground {
COLOR: #FFFFFF;
 MARGIN-TOP: 0px;
 PADDING-TOP: 0px;
 FONT-SIZE: 10pt;
 FONT-FAMILY: Verdana;
}
A:active
{
COLOR: #9999ff; text-decoration: none
}
A:link
{
COLOR: #9999ff; text-decoration: none
}
A:hover
{
COLOR: #9999ff; text-decoration: underline
}
A:visited
{
COLOR: #ccccff; text-decoration: none
}
.ContentBoxBody {
BACKGROUND-COLOR: #000000;
COLOR: #FFFFFF;
PADDING-LEFT: 5px;
PADDING-RIGHT: 5px;
FONT-SIZE: 10pt;
FONT-FAMILY: Verdana;
 BACKGROUND-IMAGE: url('');
}
TD
{
 FONT-FAMILY: Verdana;
 FONT-SIZE: 10pt;
}
BODY {
SCROLLBAR-BASE-COLOR: #000000;
SCROLLBAR-ARROW-COLOR: #FFFFFF;
BACKGROUND-IMAGE: url('http://img84.imageshack.us/img84/8403/darkmoonvr4.jpg');
BACKGROUND-REPEAT: no-repeat;
BACKGROUND-ATTACHMENT: fixed;
BACKGROUND-POSITION: center bottom;
BACKGROUND-COLOR: #000000;
FONT-FAMILY: Verdana;
FONT-SIZE: 10pt;
}.toolTipHeader {
color: lime;
font-weight: bold;
}
IMG
{
BORDER-WIDTH: 0px 0px 0px 0px;
}
HR
{
COLOR: #666699;
}

Awesome!  Thanks for the help!  Going to try it now.

I took the code above and put it on my test site to play around.  Are you wanting something like this?

http://www.guildportal.com/Guild.aspx?GuildID=139339&TabID=1184545

If so, put the following in the body section of the CSS:

OPACITY: 0.7;

Think of the "0.7" as a percent.  1 being solid, and 0 being completely transparent.


Edit:  I also have the page set to 950px wide in case your wondering why it might be skinnier (depending on what resolution you are viewing at).

Yeah, I've not figured out how to make just the background be slightly transparent.  The way I posted above makes everything in the boxes transparent.  Its the closest I've come  If you figure something else out, I'd be very interested.