I think it’s as simple as adding this to the </ head> tab of the theme:
<script type="text/discourse-plugin" version="0.8.18">
if(api.getCurrentUser() != null) {
$("#logged-in-user").text(" " + api.getCurrentUser().username);
}
</script>
And then adding a <span> to the < header> tab where *USERNAME* is:
<h2 class="x-title">Hey<span id="logged-in-user"></span>! Bienvenue dans ta nouvelle communauté</h2>