In my opinion linux mint is most customizable . I have customized it like Windows xp,7,vista,10,Mac os . it has many themes that you can make you own theme . In Linux not only you can change the theme, but you can change the window manager or even code your own.

How do I change the appearance of Linux Mint?

To access the desktop appearance settings, go to Menu > Preferences > Appearance or Menu > Control Center > Personal > Appearance. The window that opens shows three basic tabs which are Themes, Backgrounds and Fonts.

Is Linux fully customizable?

Linux is so customizable you can strip down a Linux operating system down to 50 Megabytes and still be fully functional.