Debut theme image with text overlay section height

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Hi guys, I'm having an issue with my debut theme. I'm using an "image with text overlay" as a banner on my home page so I uploaded just a photo and removed all the text . When I set the layout to "fixed width" and set the section height to "adapt to image", there is always an empty overlay covered my photo(so it darkens my photo). Does anyone know how to get rid of this empty overlay? Any suggestion is appreciated.