You can change the background color and the header image under the "Settings" tab in Multimedia Manager.
In the "Player Style" section, use the "Player Background Color" field to set the background color for the Media Player itself. Depending on the resolution and aspect ratio of the media that's currently playing, the player background may be visible on either side of the image or video (some widescreen videos may cover the player background entirely, however). If you leave this field blank, the background will be transparent and the page background color will be visible, instead.
The "Page Background Color" field controls the background color of the page on which the player is displayed. This can be seen behind the header image above the media player (provided the header image is narrow enough to allow the page background to be seen - a wide header image might obscure the page background.)
Both fields accept hexadecimal color values. For more information on hexadecimal color notation, see the Knowledge Base article Hexadecimal color values.