Help > Toggle development tools opens up a panel that lets you look at the elements / classes.
Use the “pointy-thing” tool at the top left of the Development Tools to point at the bits you want to change. This locates the item in the main Development Tools window.
I found that I could change the header cell background by using:
table th {
background-color: #FF0000;
/* sets table header cell background colour */
}
I only used Red (#FF0000
) as the background colour to make it stand out. This is not a design suggestion!!!