I am working on modifying and existing theme, but I can’t find where the background color for the find/replace is defined, or how I can otherwise set it to a different color (very light gray).

Anyone?

You probably have a .sublime-settings file in some folder of your existing theme.
It is named “Widget - .sublime-settings”, where is the name of your sublime-theme file.
So, this file contains the setting “color_scheme”, with its value pointing to a .stTheme file defining the color you’re looking for, as well as others.

Can this file be overridden in the “Packages/User/Theme - ThemeName” folder like other parts of the main theme? I tried mirroring the folder structure and put the widget file there, but it doesn’t seem to work.

I might need to just modify the actual theme.