How to make visible border lines of table header with z-index? (v.4)

Created by: MikaelBox

Hi. Often I use tables with class .table-bordered and table header with class .thead-light. Sometimes, I should use sticky headers of my tables. I make it with thead class .sticky-top (<thead class="sticky-top">). All works fine, but border lines of table header isn't shown. This behavior is linked with z-index, I guess. What can I do that make border lines of table header visible too with .sticky-top class?

Example here