The last update of this theme is at 19 september 2019, I think it's the Firefox update it provide this change. I'm sorry I can't do that because it's my first theme and I created it directly on the firefox website (you haven't the possibility to choose the color for everything like background color on new tab page for exemple). For my all other themes I use a configuration file that it allow to have more configurations than the Firefox website. But I never change the background color for the new tab page, maybe it's a Firefox configuration that put the theme as dark I don't know.

Thank you for your feedback, I fixed the problem by duplicating the image.