Hi Mark,
the content sits inside this div .inside-page-header
– the size of which is defined by the page header padding, so you can style it like so:
.generate-page-header .inside-page-header {
background-color: rgba(255,255,255,0.5);
}
If you want more control over it then you could wrap your content in a div and style that, something like this:
<div class="my-class">
HTML Markup
</div>
.my-class {
display: inline-block; /* Will fit to size of content */
padding: 10px;
box-sizing: border-box;
background-color: rgba(255,255,255,0.5);
}