I have a dnn website with BLACK background, when i'm taking the text editor & container settings all fonts are in balck colour, nothing is visible. Anybody tell me how to solve this issue????

The problem is with the skin you have assigned for "admin" sections of the site. If you switch the admin skin to the standard DotNetNuke skin, this issue will disappear.

To fix it for your skin, you'll need to identify the CSS class that causes this and change it. It will be in your skin.css file.

where i can find the settings for admin skin? Please help...

admin menu / site settings you can set the skin for your portal and admin pages

There is another work around to this however and that's to inform the FCK Editor to use a different class for it's body and you then specify the class details in your skin.css file, we cover this in details here:

FCK Editor CSS Skin Styles Preview in DotNetNuke

Look at video 2

This means that you can then see the exact stylings to match the layout of your page skin rather than an admin skin.