Fixing bootstrap dark-table top border (alshedivat#1763)

This addresses alshedivat#1425.

Contributions:
- Added border attribute to `.table-dark` within `_base.scss` to
override bootstrap theme border settings for `.tabled-bordered`.
Displays top border in dark mode for only those tables that are bordered

_sass/_base.scss

@@ -50,6 +50,9 @@ table.table a {
 
 .table-dark {
   background-color: transparent;
+  &.table-bordered{
+    border: 1px solid var(--global-divider-color) !important;
+  }
 }
 
 figure,