Since the default Libreoffice color scheme is light gray (15%ish?), it is very difficult to see selected tab(s) since they turn white. Easiest fix would be to have default tab color that is something closer to a 50-60% gray
Agree that contrast of the active Tab could be a bit higher for indicating the selected active sheet. Current gray (inactive) and White (active) is a little muddled--but not that unreadable. So for simplicity it might be appropriate to use the same--color and reversed font color--used for the selected active Column or Row on the active sheet Tab. Default for those now is a pseudo Blue gradient (just 2 hues), and font color toggled white when active.
*** This bug has been marked as a duplicate of bug 34233 ***
*** This bug has been marked as a duplicate of bug 104564 ***