Ok in the GameX.html.template in the same folder, there are
.wrapper{
position: relative;
margin: 1em auto 10px auto;
text-align: center;
min-width: 640px;
min-height: 480px;
max-width: 95%;
display: block;
align-items: center;
position: relative;
text-align: center;
justify-content: center;
}
and
#canvas:not([fullscreen]) {
padding-right: 0;
margin-left: auto;
margin-right: auto;
width: 100%;
}
Changing these values works great.
Thanks for showing me the way, svv3dUDN !