How to add an overlay on the image of the stellar layout?

Copy the below snippet to your CSS file.

.el-single-post-template-stellar .el-featured-image:before {
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    content: '';
    background: rgba(0,0,0,0.5);
}