You don’t need the media="all" bit.

Also it is better not to repeat properties that stay the same so:
#buddypress div.item-list-tabs#subnav {
background: #0E0C0C;
}

Should work.

It certainly works in chrome on your website. Do you see it in chrome?

I don’t know what you mean by your ‘theme css box’? If you mean that your theme has…[Read more]

Select the developer tools window.

In the bottom left corner of the developer tools window you should see a magnifying glass icon. If you hover over it you get the message: select an element in the page to select it

Once selected if you hover over items on the page it will show you extra details.

Click on an item, such as a page title…[Read more]

http://www.w3schools.com/css/default.asp

Try looking at the html of the webpage using the developer tools in Chrome, Safari or Internet Explorer, or use Firebug with Firefox, or Opera’s Dragonfly…