This will change the general design of various theme elements. Use dark or light. Determine your light color and your dark color. That's it.
What are light and dark colors?
These are for example the background of the shop, light accents, etc. If you turn it around, you will mainly see the dark color. Play with it and see what it does.
If you use a dark theme, you will need to use transparent .png files as images for your products. Or the background color of your product photos should adjust to the dark color of your theme.