Hello There,
Thanks for writing in! It is not fully fullwidth because you have this custom css code in your customizer,
.blog .masthead + .x-container.max,
.single-post .masthead + .x-container.max {
max-width: 700px;
}
I would suggest that you update it and use this instead:
.blog .masthead + .x-container.max, .single-post .masthead + .x-container.max {
width: 100%;
max-width: 90%;
}
Hope this helps. Please let us know how it goes.