Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

$acolor: rgba(#040708, 0.76);

@function rgba_blend($fore, $back) {
  $ored: ((1 - alpha($fore)) * red($back) ) + (alpha($fore) * red($fore));
  $ogreen: ((1 - alpha($fore)) * green($back) ) + (alpha($fore) * green($fore));
  $oblue: ((1 - alpha($fore)) * blue($back) ) + (alpha($fore) * blue($fore));
  @return rgb($ored, $ogreen, $oblue);
}  

$acolor: rgba(#040708, 0.76); // build a test color with alpha

#a_div {
  background-color: rgba_blend($acolor, white);
}

SassMeister-output.css

#a_div {
  background-color: #404343;
}