Unfortunately, customizing certain aspects of Eclipse theming remains tricky. While you can adjust most colors, changing the color for this specific element isn't currently possible. Part of the reason is that the eclipse UI implemented differently across operating systems (Linux, Windows, macOS). However, the Initiative31 to unify and modernize that. On Linux, the color of the selected element appears to be determined by the operating system's theme, on windows (which you seem to use) this gray color seems to be fixed.

You could use a theme with another background to bake the selected file better visible. I recommend trying the Darkest Dark. Let me know if this solved your problem.