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please contact the developer of your theme for help with your question –

http://codex.ww.wp.xz.cn/Forum_Welcome#Commercial_Products

If you’re not using a Child Theme, and if the theme doesn’t have a designated section of the dashboard for CSS modifications then install this Custom CSS Manager plugin http://ww.wp.xz.cn/plugins/custom-css-manager-plugin

Then use its “CSS Code” section of the dashboard to hold your CSS modifications:

Alternatively use your Child Theme style.css file to hold your CSS modifications:

.site-header {
    text-align: center;
}

.site-logo {
    float: none;
}