You can make the definition of the cropping and focus area of the folder theme image more user friendly by adding an 'Edit' button to Folder Theme image box (or less user friendly a context menu) .
If you click that button, the theme image should be displayed in a larger window like you see if you open an image in the explorer. Next you can use the cursor to define the area you like to use as theme image as is already done in the 'Cut the edges off to improve your composition' window You have to add a scale factor to get a theme image of the size you want, so for example 3840x800 instead of 1920x400
With this implementation you can remove the Sample shape options in the explorer Set crop focus... pop-up and use that pop-up only for the thumbnail focus, the user has no longer to search for the corresponding theme image thumbnails as is required in the current implementation.
You can also remove the other folder theme image context menu's by adding 2 extra buttons in the opened window.