in bootstrap, it seems that nested rows gives me a small additionnal right margin like this:
by definition a row is :
margin-right : -15px;
margin-left : -15px;
<div class="row">
<div class="customize">
<div class="row">
<div style="display:flex; margin-top:50px;">
<h2 class="slide-title">Personnalisez votre vitrine CINETIK</h2>
</div>
</div>
Use class=col-md-12 like this:
<div class="row">
<div class="customize">
<div class="row">
<div class="col-md-12" style="display:flex; margin-top:50px;">
<h2 class="slide-title">Personnalisez votre vitrine CINETIK</h2>
</div>
</div>