I want to achieve this using html and css:

I have tried to set the opacity of the container
Try using rgba as a 'pre content' overlay to your image, its a good way to keep things responsive and for none of the other elements to be effected.
header #inner_header_post_thumb {
background-position: center;
background-size: cover;
position: relative;
background-image: url(https://images.pexels.com/photos/730480/pexels-photo-730480.jpeg?w=1260&h=750&auto=compress&cs=tinysrgb);
border-bottom: 4px solid #222;
}
header #inner_header_post_thumb .dark_overlay {
position: relative;
left: 0;
top: 0;
right: 0;
bottom: 0;
background-color: rgba(0, 0, 0, 0.75);
}
header #inner_header_post_thumb .dark_overlay .container .header-txt {
padding-top: 220px;
padding-bottom: 220px;
color: #ffffff;
text-align:center;
}
header #inner_header_post_thumb .dark_overlay .container .header-txt h1 {
font-size: 40px;
color: #ffffff;
}
header #inner_header_post_thumb .dark_overlay .container .header-txt h3 {
font-size: 24px;
color: #ffffff;
font-weight: 300;
}
header #inner_header_post_thumb .dark_overlay .container .header-txt p {
font-size: 18px;
font-weight: 300;
}
header #inner_header_post_thumb .dark_overlay .container .header-txt p strong {
font-weight: 700;
}
See a working codepen here