Most designers use #RRGGBB or #RGB notation for colors in HTML and CSS. CSS has another option which allows you to also specify how opaque or transparent that color is, namely rgba() notation.
The rgba() specifier takes 4 arguments: red, blue, green and alpha. Red, blue and green are numbers from 0 to 255 in decimal (not hexadecimal), while alpha is a decimal number between 0 (totally transparent) to 1 (totally opaque). You can use this notation by typing it manually in the Header Background menu (for instance), or by entering it into one of your CSS rules.