css horizontal line appearing

[ Moderator note: moved to Fixing WordPress. ]

Hi,

I have a static WP site https://edinatuition.com

There is a horizontal line appearing just below the header section of my pages. It wasn’t there originally and I cannot find it in my code.

<header id="header" role="banner">
		  <h1 id="site-name"><a  title="Edina Tuition" rel="home">Edina Tuition</h1><div id="header-message">Maths tuition Edinburgh Scotland since 2012</div><nav id="nav" role="navigation"><div class="menu"><ul id="menu-edina-tuition-1"><li class="menu-item menu-item-type-post_type menu-item-object-page menu-item-710">About
<li class="menu-item menu-item-type-post_type menu-item-object-page menu-item-959">What they’re saying
<li class="menu-item menu-item-type-post_type menu-item-object-page menu-item-has-children menu-item-1685 has-children">Study tips
<ul class="sub-menu">
	<li class="menu-item menu-item-type-post_type menu-item-object-page menu-item-1674">Think like a mathematician


<li class="menu-item menu-item-type-post_type menu-item-object-page menu-item-730">Links
</div><select><option value="">- Navigation -</option><option value="https://edinatuition.com/about/"> About</option><option value="https://edinatuition.com/what-they-are-saying/"> What they’re saying</option><option value="https://edinatuition.com/study-tips/"> Study tips</option><option value="https://edinatuition.com/think-like-a-mathematician/">- Think like a mathematician</option><option value="https://edinatuition.com/links/"> Links</option></select></nav>		</header>

There is a ::before and ::after appearing immediately after the opening header tag and immediately before the closing header tag respectively and I don’t understand these.

Can anyone help me solve this mystery?

Thanks.