If you are an Ubuntu user like me, then you could appreciate the details posted at the Ubuntu forums. I’m definitely bookmarking this one for reference!

A lot of things, actually. In this HowTo I will cover:
- Widgets: this are the little elements like buttons, loading bars, menus…, which are used in your applications
- Window borders: the part of the window containing the buttons to close, maximize and minimize
- Icons
- Mouse cursors
- Panels
- Splash screen: this appears when you login before the desktop os ready
- Login manager: the application which greets you after the boot
- Desklets: little apps which show useful information over your desktop
- Fonts
- Grub: the application which lets you choose what operating system you want to boot
- Usplash: what appears when the system is loading at boot

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Read about Theming your Ubuntu Desktop