Transparent background color for main section – on home page only

I read the following forum posting which describes how to make the background color of the main section transparent – which is fantastic.
https://kriesi.at/support/topic/transparency-opacity-of-background-header-main-footer

What I am trying to find out is how to target the following CSS to just my frontpage (the page as set in Theme Options -> Frontpage Settings):

.main_color {
background-color: rgba(255,255,255,.9);
}

#main, .html_stretched #wrap_all {
background-color: transparent;
}

Thanks!