you will need to use rgba color codes to do that, here is an example:
<style>
main{
background-color: rgba(255, 255, 255, 0.3) !important;
}
</style>
255 is the color for white, so this will make the background white and slightly transparent. changing the numbers (255, 255, 255) will change the color of the background, and changing the 0.3 will change how transparent it is. google "rgba color codes" to get the numbers for the color background you want.