From https://developer.mozilla.org/en-US/docs/Web/CSS/background-color
To set background color:
/* Hexadecimal value with color and 100% transparency*/
background-color: #11ffee00; /* Fully transparent */
/* Special keyword values */
background-color: transparent;
/* HSL value with color and 100% transparency*/
background-color: hsla(50, 33%, 25%, 1.00); /* 100% transparent */
/* RGB value with color and 100% transparency*/
background-color: rgba(117, 190, 218, 1.0); /* 100% transparent */