Code when highlighting is not accepting

Hi, I had this code added to custom codes. This is to change color when highlight text.

::selection {
background: #d4e6fc;
color: black;
}

Initially it worked fine. But after highend updates, it stopped working.
Then I put that code in Highend Options>general settings (see the attached pic). It worked fine for a while too. Now it stopped working again.

I asked Bluehost paid service to help me to fix it (I have the paid service). They are unable to fix it either. They think it could be a theme issue.
Can you help me to activate this code please?