Horizontal Line in Dark Mode Does Not Appear #2209

Describe the bug
When using Bookstack in dark mode and you use the "horizontal line" button when building your page it places a horizontal line in your page but you are unable to see it as long as you are in dark mode.

Steps To Reproduce

1. Turn on dark mode
2. Create a new page
3. Place horizontal line via the button right below the page name
4. As you can see its not viewable. Change to light mode or export a Contained Web File and it will appear

Expected behavior
For the horizontal line to be visible in dark mode

Screenshots
Dark Mode

Light Mode

Your Configuration (please complete the following information):

• BookStack v0.29.3
• PHP 7.3.18 (cli) (built: May 15 2020 16:10:31) ( NTS )
  Copyright (c) 1997-2018 The PHP Group
  Zend Engine v3.3.18, Copyright (c) 1998-2018 Zend Technologies
• Docker

Additional context
N/A