linear-gradient is an image, so images (background and gradient) must be comma-separated
.header { height: 670px; width: 100%; background-image: linear-gradient(to right, rgba(9, 5, 47, .85) 0%, rgba(49, 29, 94, .85) 100%), url(../img/header_bg.png); }
Further, if I understood correctly about the issue with the text, then:
Create a block and assign it such a class, filling the text with an example:
<div class="header"> Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris enim enim, semper sed libero placerat, venenatis eleifend velit. Aliquam imperdiet nunc in libero condimentum viverra. Quisque eu est et mauris pellentesque vulputate. Curabitur posuere sollicitudin urna id malesuada. Vivamus tincidunt tristique diam, id porttitor neque tempor at. Quisque eget enim in tortor scelerisque vehicula vel eget justo. Nunc rutrum iaculis tortor at porttitor. </div>
JSFiddle example .