Backport Reboot's th updates

Manually backports #30781 to v4.

scss/_variables.scss

@@ -363,6 +363,7 @@ $table-border-color:          $border-color !default;
 
 $table-head-bg:               $gray-200 !default;
 $table-head-color:            $gray-700 !default;
+$table-th-font-weight:        null !default;
 
 $table-dark-color:            $white !default;
 $table-dark-bg:               $gray-800 !default;