Remove Header Code from Unlinked Page Issues

I am a musician and audio engineer... not a coder! So your assistance is sincerely appreciated.

I am trying to hide the Header on a single Unlinked page (No Navigation)

I create a code block below the navigation section and enter the following:

<style>
  header#header {
    display: none;
}
  footer#footer {
    display: none;
}
</style>

This appears to work after saving the page, but when returning to the page much of my data is not visible - but is again upon entering EDIT page content mode.

Is the code block moving? and if so how do I get around this?

Many thanx

Well I believe I solved this by creating a LINE block first, then the code block.

Testing appears as expected now.