table: Add border color relative to theme for accessibility (#25755)

scss/mixins/_table-row.scss

@@ -1,6 +1,6 @@
 // Tables
 
-@mixin table-row-variant($state, $background) {
+@mixin table-row-variant($state, $background, $border: null) {
   // Exact selectors below required to override `.table-striped` and prevent
   // inheritance to nested tables.
   .table-#{$state} {
@@ -9,6 +9,15 @@
     > td {
       background-color: $background;
     }
+
+    @if $border != null {
+      th,
+      td,
+      thead th,
+      tbody + tbody {
+        border-color: $border;
+      }
+    }
   }
 
   // Hover states for `.table-hover`