You can change course themes from anywhere in your course using the Change Course Theme function. Point to the icon to view a drop-down list showing all the available course themes, and click one to select it. Scroll through the theme preview box and select a theme.
The course theme changes immediately to the new choice. To change the theme, select another.
Style options control the appearance, theme, course menu style and layout, content appearance, course entry point, and the banner image for a course. On the Control Panel, expand the Customization section and select Teaching Style to access course style options.
Course themes add a background image to the course display and change the color of the menu, buttons, and controls. Use the scrollable list to select the appropriate course theme from the thumbnail sample images. You can change the theme again at any time. Themes do not affect course content or a chosen course structure.
Complete Details from Blackboard Here