Add Customization

Customization.md

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+As of writing, pi has no native way of customizing it.
+The [Fyne Demo](https://github.com/fyne-io/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](https://docs.fyne.io/started/) for more information.
+
+This is how the installation will look for *NIX systems:
+```bash
+go install fyne.io/fyne/v2/cmd/fyne_demo@latest
+```
+
+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](https://go.dev/doc/install).
+
+Run the command as follows:
+```bash
+fyne_demo
+```
+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.