Separator (hr) elements in wiki page should be darker

Summary

When adding a separator in a markdown wiki page, saving and viewing the page, the separator is in very light gray and really hard to see.

Steps to reproduce

Edit a wiki page, and add a separator within the page (markdown syntax) :

some text

---

some other text

After saving, the page includes the separator, but it is very light and hard to see.

Expected behavior

It should be in a darker shade of gray.

Actual behavior

Too light shade of gray.

Relevant logs and/or screenshots

Separator_is_not_very_visible

Output of checks

Happens on my installation at work (GitLab Community Edition 8.15.2 790035fc)

Possible fixes

I don't have the problem when editing a wiki page and accessing the Preview pane:

Within_the_preview_pane

When checking the differences in CSS, I see an additional style applied to the preview pane, defined here:

https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/stylesheets/framework/markdown_area.scss#L111