Hi there!
Thanks for reaching out to us.
Your site URL would be better to have. The demo of finion is looking good, both Header and main body container are same width.
However, the following custom CSS may help to set the container width of the main body:
@media (min-width: 1200px)
#sp-main-body .container {
max-width: 1140px;
}
You can change the width value as per your need.
Here is all about custom code placement in Helix Ultimate: https://www.joomshaper.com/documentation/helix-framework/custom-code-css-js-meta
Best Regards