<button>
по центру блока div
? <style>
.center {
text-align: center;
}
</style>
<div class = "center">
<button id = "button_reg">Регистрация</button>
</div>
<style>
.Absolute-Center
{
margin: auto;
position: absolute;
top: 0; left: 0; bottom: 0; right: 0;
}
</style>
<div class = "Absolute-Center">
<button id = "button_reg">Регистрация</button>
</div>