Styling the Archive page

You can do this by 2 ways…
With HTML in your template or CSS in your style sheet.

To place 2 thin lines using HTML code, use the HTML <hr> tag like this :

<hr>
{Your WordPress PHP Meta}
<hr>

Or using CSS by adding border-top:1px solid white and border-bottom:1px solid white in your WordPress PHP Meta container.

Try using a browser developer tool for this kind of exploratory CSS work.
I use Google Chrome’s built-in Developer Tool and it’s really easy to expose the webpage’s underlying HTML and CSS.

Now that you know the style, from being exposed by the Developer Tool, you can add a new style to override it.

You ought to add new styles externally, not within the theme’s files. Changes to the theme’s files will compromise the theme and be compromised when the theme updates.

External changes can be made using a Child Theme or through a plugin that provides Custom CSS functionality.

I recommend using the Jetpack plugin and its Custom CSS functionality to hold your CSS modifications.