Deleting content background-color for a single page

Hi there,
I struggle with deleting the background-color of the content-container for a single page.
If I set the background color to transparent in the customizer, then of course it sets the background transparent on all pages. I would like to set the background for the content only on the first page.
I tried this in the additional CSS (ans several others...):
.page-id-99 #content .site-content {
background-color:rgba(255,255,255,0);
}
I also tried background-color:none;
But it makes no changes :( What do I miss here?
Thanks, Erika