I've 3 stores in Magento 2.1 using the Magento Luma theme.
But, I want the CSS to be different for the 3 stores.
Is ther any way I can copy the the Luma theme and rename it, so that I can make the modigifcaions different for each store?
Thanks for a quick answer
You must have to create single theme first based on parent luma theme.
You can check this thread to create new theme in magento 2 parent as luma,
Now For second and third store create theme which extends Your current theme(above created theme) as parent theme and keep only required files and do your changes.
If issue resolved, click kudos/accept as solutions.