It looks like you're not understanding your CSS. Have you just copied it from another website and applied it to your own screen?
@media screen and (max-width: 768px) {
..
.smart-table thead{
display: none;
}
..
}
What this is saying is "when we are rendering for a screen
and the view width is 768px or less: hide the
thead
element". If you want the header to stay then you need to remove the
thead
CSS style.
CSS @media Rule[
^]