Hi Wendy-
It doesn’t have to be one of the palette colors, you can choose. Hydra’s a little tricky because it uses JavaScript to change the background as your visitor scrolls down the page, so you’ll probably want two rules for this.
For the background before you scroll down, use this rule:
.palette-primary .site-header { background-color: rgba( 62,62,62,0.8 ) !important; }
The rule above is using the default color palette’s third color of #3e3e3e converted into RGBA so that the transparency of the original design is preserved.
For the background after you scroll down, use this rule:
header.smaller { background-color: #3e3e3e !important; }