Missing border at the bottom of docextrapanes

Steps to reproduce

1. Login as Admin
2. Go to Administer Wiki > Look & Feel > Themes
3. Click 'Customize' to edit the theme (tested on default Iceberg theme)
4. On "Advanced" section, set the @xwiki-border-color to purple by adding:

   @xwiki-border-color: #ff00ff;

1. Save and open the Main.WebHome page in View Mode
2. Observe the docextrapanes borders

Expected results

The docextrapanes have a purple bottom border (on pages with little content)

Actual results

No border is present at the bottom of docextrapanes.

The issue could not be reproduced on XWiki 15.9, 15.5.4 or 14.10.20.