Found in style.css in the current theme directory (wp-content/themes/{theme name}/style.css)
My added code in red.
.aligncenter {
display: block;
margin-left: auto;
margin-right: auto;
}
.alignleft { float: left; padding-right:10px; }
.alignright { float: right; padding-left:10px; } -- Note the added padding styles to account for use images not running into wrapped text
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