Why is the CSS horizontal alignment for divs inside div container not displayed in my WP Theme?

Never add a padding on something whose width/height you fix. Some browsers add the padding to the width/height; others don’t. Use an inner div to work around the issue:

<div style="width:200px; height: 200px; background: red; padding: 10px;">
220x220
</div>

<div style="width:200px; height: 200px; background: green;">
<div style="padding: 10px;">
200x200
</div>
</div>