Clearly I'm making some sort of mistake here, but in Design > Custom CSS I'm finding that the following is applying globally to the entire site, rather than just one section on the homepage. The ID is correct.
What's my mistake, please?
#block-yui_3_17_2_1_1605803997792_3851, .Main-content {
padding-top:0px;
padding-left:0px;
padding-right:0px !important;
width:100%;
}