I'm not sure if this is the right forum for this kind of thing, but I've noticed that the Header bar in Scrivener is always the same white color, no matter what theme I have selected. I'm not sure if this is specific to my computer or not, but if it's not it might be more visually appealing to have the header's color change to match the theme.
Windows apps take their window color from the active theme. Active apps have one color (in your case it looks like it's white) and inactive apps a different color (so you can tell which one is the active app). You can easily confirm if this is the case by opening another app and arranging the window so you can see Scrivener in the background. If the header bar you're talking about changes color when the other app is open, and goes back to white (and the header of the second app changes to the same non-white color when Scrivener is active), then this is all as expected and is controlled by your local Windows desktop theme settings.
-- Devin L. Ganger, WA7DLG Not a L&L employee; opinions are those of my cat Winner "Best in Class", 2018 My First Supervillain Photo Shoot