Sidebar is in the wrong place when desktop, but not when mobile - html

Question

I have a problem with the website I'm currently making for my art
theyoungillustrations.com
I have absolutely no experience in coding, so please explain like I'm five
I'm altering a excisting code, but I experience a problem with the sidebar. Because I made a banner (at the top), the sidebar is currently under it and I can't find out how I can fix that without ruining the mobile-sidebar.
This here seems to be the target, but the style-ing part is the one I'm unsure of.
<div id="sidebar" class="{select:Header Text Case} ">
<div class="inner animated slow">
<div class="wrap">
<nav class="sidebar-navigation cl">
<ul>
{block:IfHomeLink}
<li>Home</li>
{/block:IfHomeLink}
{block:HasPages}
{block:Pages}
<li> {Label}</li>
{/block:Pages}
{/block:HasPages}
</ul>
</nav>
</div>
</div>
</div>
<div id="header" class="{select:Header Text Case} ">
<div class="wrap">
<div class="inner">
<div class="logo">
{block:ifnotlogoimage}
{Title}
{/block:ifnotlogoimage}
{block:iflogoimage}
<img src="{image:Logo}" alt="logo" />
{/block:iflogoimage}
</div>
<nav class="navigation cl">
<ul>
{block:HasPages}
{block:Pages}
<li> {Label}</li>
{/block:Pages}
{/block:HasPages}
</ul>
</nav>
</div>
</div>
<div class="sidebar-toggle">
Menu
</div>
<div class="close-mobile-modal">
<svg width="16px" height="20px" viewBox="0 0 20 20" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<g stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
<path d="M7.172,10 L3.642,6.47 L0.102,2.93 L2.928,0.1 L10,7.173 L17.07,0.103 L19.9,2.93 L12.827,10 L19.897,17.07 L17.07,19.9 L10,12.827 L2.93,19.897 L0.1,17.07 L7.173,10 L7.172,10 Z"></path>
</g>
</svg>
</div>
</div>
</div>
</div>
Then I have this:
}
#sidebar h3 {
margin-bottom:20px;
color:{color:Header Text};
font-size:13px;
}
#sidebar-navigation {
color:{color:Header Text};
background:{color:Header Background};
font-size:{text:Header Size};
}
But I'm clueless. Thanks for helping.

Answer 1

This is because you need to add a breakpoint for various devices ,you can take help of this url :https://www.w3schools.com/css/css_rwd_mediaqueries.asp