You put the theme anywhere you like, and import it.
The picture, you place in the permanent location of your choice.
My directory: D:/Writing/Scrivener Themes. (Substitute drive letter for the drive where you keep your files).
After choosing “Import Theme” in Scrivener, open Window—>Themes again, and you will see Midnight located by itself below the standard themes that come w-Scrivener.
Select it, then close Scrivener completely, and reopen.
Scrivener will have moved the theme to the app data folder where it keeps them.
It will not have moved in the picture, as it is set to let you change those anytime you want.
To use the picture, open Options, main editor & go to the color tab. You will find an extra button to choose a pic. Select it, click OK.
I’m on a phone right now or I would give you the buttons name. It is right above the button where you choose your color.
Lastly; it has my preferences saved with it, and you may not like those. So please do go through the options tab and change things to your own liking.
If you happen to reset a color for an option, there’s a great free little app for Windows called Color cop, which will let you change it back via eye dropper.
I know—a pain, but in order to have the default be what the theme is; I would have needed the source code of Scrivener decompiled.
It is a really quick fix with Color Cop or similar eye dropper program though.