As of writing, pi has no native way of customizing it.
The Fyne Demo application can be used to configure the global Fyne theme, which will change pi's theme.
Installing the Fyne Demo
You must have Go installed, along with a C compiler and the necessary dependencies required for building graphical applications. Refer to the Fyne documentation for more information.
This is how the installation will look for *NIX systems:
This will install the Fyne Demo into your GOBIN environment variable. This should be in your PATH. If it isn't, then follow the Go documentation.
Run the command as follows:
Suffix with .exe if you are on Windows.
In the new window that opens, hit Ctrl+Comma.
You can now change the theme, font size, animations and main color of all Fyne applications, including pi.