background-blend-mode: multiply;

Hi Guys

I am trying to use the CSS blend modes on my wp theme but I just can’t get it to work.

I want to apply it to a hovered thumbnail using this code:

img.postthumb:hover {
  background-color: #01157c;
  background-blend-mode: multiply;
}

I am adding it to the CSS Stylesheet Editor but the background-blend-mode: multiply; appears red suggesting WP doesn’t know that it is code and when I save it it just disappears.

What am I doing wrong?