Remove the .style after choosefanpagebtn:
var choosefanpagebtn = window.opener.document.getElementById("choosefanpagebtn");
choosefanpagebtn.className = "green_button";
Should work. Not tested, let me know. Also, this line was unnecessary:
choosefanpagebtn.className = "";
Because you're not doing +=. If somehow you did that, and then
choosefanpagebtn.className += "green_button";
later, I could see that line being useful.