In computing, a theme is a preset package containing graphical appearance details. A theme usually comprises a set of shapes and colors for the graphical control elements, the window decoration and the window. Themes are used to customize the look and feel of a piece of computer software or of an operating system. In computing, a theme is a preset package containing graphical appearance details. A theme usually comprises a set of shapes and colors for the graphical control elements, the window decoration and the window. Themes are used to customize the look and feel of a piece of computer software or of an operating system. Themes are often used to change the look and feel of a wide range of things at once, which makes them much less granular than allowing the user to set each option individually. For example, you might want the window-borders from a particular theme, but installing it would also alter your desktop background.