BTCPay Server is built on Bootstrap, this gives you access to lot's of custom theme maintained by the community. Browse Bootswatch to find some themes.
Let's see how we can take advantage of it.
Create a new store
Setup a derivation scheme for BTC for this store
Create a new Point of Sale app attached to this store (in the Apps menu)
You will see that the default design for such point of sale is:
You can copy the link to the theme:
Now, go into the settings of your store and paste the link to customize the bootstrap theme:
And enjoy a new theme!