Remove thead background (#18883)

* Remove thead background

* Update tables.scss

Author: heiskr

stylesheets/tables.scss

@@ -34,7 +34,7 @@
     vertical-align: top;
     z-index: 1;
     top: 0;
-    background: var(--color-bg-tertiary);
+    background: var(--color-bg-primary);
     box-shadow: 0 3px 0 0 var(--color-auto-gray-4);
     padding: 12px 8px;
     border: 0px;