Hi Murali,
Couple ways to modify those elements for a specific page. You can either find the page id and add it to the CSS, so something like this:
.page-id-xx h1 {
background-color: #ffffff;
font-weight: 700;
line-height: 10px;
}
Or use the Simple CSS metabox on the page you want to modify, then you don’t need the .page-id-xx
selector.
There shouldn’t be conflicts between Simple CSS and Custom CSS but I would recommend to stick with one since they basically do the same thing.
Let me know if this helps.