just add this style for each of your textarea tag
style="background:rgba(0,0,0,0.5);color:#00ff00"
full code from what you gave above will be
<center>
<body>
<textarea style="background:rgba(0,0,0,0.5);color:#00ff00" name="" id="origin" cols="150" rows="20" onchange="replace()" onkeyup="replace()"></textarea>
<br>
<textarea style="background:rgba(0,0,0,0.5);color:#00ff00" name="" id="target" cols="150" rows="20"></textarea>
</center>