As you have just discovered, the opacity property applies to everything in the element. To make just the background translucent, use the rgba colour format for the background-color property.
The videos in the tutorial were created using the original release of Dreamweaver CC. The latest version of DW CC (13.1) has a new colour picker, which has an opacity (alpha transparency) slider.
Select the RGBa button at the bottom of the colour picker to change the colour format, and then drag the Alpha slider down (both highlighted in the preceding screenshot).