Soooooooo what's your question? Do you want to know how FileMaker did this or why FileMaker did this? Seems to be the default method for all themes--tab you click on is one color, label not clicked is the other. Are you trying to make it so both labels are the same color as the panel?
Try right clicking in the Inspector, Fill>None for each tab panel, maybe that's more of what you're looking for. Then you can go back to Fill> Solid Color, and the labels should all be the same
PS. Usually when people ask for help they say "please" and "thank you", instead of challenging people to answer their question. Probably why no one has responded.
my bad. please and thank you