To celebrate the arrival of Autumn you can download a set of free Windows 10 themes to the Microsoft Store.
Titled Pumpkin Season, the description of the high-quality wallpaper selection reads as follows:
It’s pumpkin time. They glow, they grin, and usher in the fun. Get 13 festive autumn images in this free Windows theme set.
After downloading the collection, go to Start > Settings > Personalization > Themes and click on the theme you prefer.
To apply and change to the most appropriate colour, based on the current wallpaper, go to Customize > Colors > Automatically select a primary colour from the background.
Find the collection in the Microsoft Store here.