In order for your main invoice table header to be green, go to Settings > Account settings > Print settings > Customize print with CSS and paste the following code:
table.main_inv_table th {
background-color: #7BB453;
border: 0 none;
color: #FFFFFF;
font-weight: normal;
}
If you paste this code in Settings > Account settings > Print settings > Customize print with CSS, all template headers will be green
.clean.to_right th,
.clean.split_three strong,
.inv_to_pay th,
strong,
.invoice_outline p strong,
.invoice_outline strong,
.split_half th,
.split_half th .buyer,
.split_half th .seller {
color: #7BB453 !important;
}