Hi Mary,

to hide the crossed background lines use

.bg-lines {
    display: none;
}

in customizer -> additional CSS

To change it I think you have to be familiar with <svg> tag:

https://www.w3schools.com/graphics/svg_intro.asp

the background lines are constructed like this:

<div class="bg-lines hidden">

        <svg>

             <line x1="20%" y1="100%" x2="80%" y2="0"></line>
             <line x1="80%" y1="100%" x2="20%" y2="0"></line>
    </svg>
</div>

in header.php (scroll to the bottom)

That's a bit more complicated.

You can for example which page you are into with is_page function

https://developer.wordpress.org/reference/functions/is_page/