It would be great if you use :hover pseudo class over the onmouseover event
td:hover
{
background-color:white
}
and for the default styling just use
td
{
background-color:black
}
As you want to use these styling not over all the td elements then you need to specify the class to those elements and add styling to that class like this
.customTD
{
background-color:black
}
.customTD:hover
{
background-color:white;
}
You can also use :nth-child selector to select the td elements