Removing masthead/menu from a specific page

I removed the masthead & menu from my landing page with the following code in “styles.css”:

/* Remove Masthead */
.page-id-19 header#masthead { display: none !important; }
/* Remove Navigation */
.page-id-19 .main-navigation { display: none !important; }

This worked for a couple of hours on several devices, but new the masthead appeared again. Why is that and what can I do about it?

Thanks