We'll consider making an easy option for this in the future. For now if you really don't like the color you could add the following piece of code at the bottom of the 'Program Files (x86)\Lansweeper\Website\js\jquery-1.9.1.min.js' file:
$(document).ready(function() {
$('#loginfirstelement').css('background', '#00ff00');
});
We advise making a backup of the file before you edit it. You'll need to press F5 on the web page to get the new results. Keep in mind that this file will be overwritten when upgrading.