navigation highlight bar

Dear Support Team,

On the website https://emmerams-muehle.de/, a transparent header is displayed at the beginning (apparently), if you scroll down it turns white.
As long as the header is transparent, a white bar is displayed below the active menu item: this is located in the middle of the bottom half of the transparent header: see: https://emmerams-muehle.de/support/header1.jpg

When the header then turns white, this bar slides down to the lower header edge. see: https://emmerams-muehle.de/support/header2.jpg

I would like the bar to be at the bottom of the header in the transparent view as well, like at https://emmerams-muehle.de/support/header2.jpg
How can I achieve this?
Many thanks for your support.
Best regards, Diana

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