You can center the standalone buttons in modern browsers by following the CSS code example below.

<style>
#my_centered_buttons { display: flex; justify-content: center; }
</style>

<div class="a2a_kit a2a_kit_size_32 a2a_default_style" id="my_centered_buttons">
    <a class="a2a_button_facebook"></a>
    <a class="a2a_button_mastodon"></a>
    <a class="a2a_button_pinterest"></a>
    <a class="a2a_dd" href="https://www.addtoany.com/share"></a>
</div>

<script async src="https://static.addtoany.com/menu/page.js"></script>

To center-align the buttons, try adding the following CSS code to your "Additional CSS" box in Settings > AddToAny.

.addtoany_content { text-align:center; }

If the above code does not center-align the buttons, you will need to customize the CSS code for your WordPress theme. For additional help, consult your theme's author or the WordPress Support forum for themes.