Default no sidebar?

This can be done universally with a few lines of CSS:

#content {
  width: 92%;
}
#sidebar {
  display: none;
}

https://startupwp.com/topic/customizing-your-theme/

However, if you need this change to actually be hard-coded in the page templates so you still have the option then we’ll need to use a child theme. If on your other support topic you decide to trek into child theming territory we can help you accomplish both in the child theme and future customizations within the child theme where you’ll want to manage customization from then on. It’s actually not as daunting as it might sound and we’ll help you every step of the way.