Hello,
1) Try to use the next custom CSS code:
.tabs .tab-title {
color: #2B5F84;
font-size: 18px;
font-weight: bold;
}
2) You can change color and bg color via Theme Options > Styling > Dark buttons.
About font:
.single-product .form-submit input[type="submit"] {
font-size: 18px;
font-weight: bold;
}
Regards