You cannot change it to curved edges without images.
To center:
Find this line in your code(without red):
Code:
var tip = '<table bgcolor="' + tipBorderColor + '" width="' + tipWidth + '" cellspacing="0" cellpadding="' + tipBorderWidth + '" border="0"><tr><td><table bgcolor="' + curBgColor + '" width="100%" cellspacing="0" cellpadding="' + tipPadding + '" border="0"><tr><td>'+ startStr + messages[num][0] + midStr + '<span style="font-family:' + tipFontFamily + '; font-size:' + tipFontSize + '; color:' + curFontColor + ';"><center>' + messages[num][1] + '</center></span>' + endStr + '</td></tr></table></td></tr></table>';
And put in the red changes I amde
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