A theme is an XML document (with the .c1theme extension when saved in a file) that contains a consistent set of values for properties determining the look and feel of supporting controls. When applied to a form in an application (at runtime and/or design time, using the C1ThemeController component), the theme changes the look of controls on that form so that they all comply with the theme.

Internally, a theme is divided into sections corresponding to different ComponentOne products, so that each product only uses its own section when the theme is applied. This allows to provide a customized "themed" look for even the smallest and most specialized details of a control. Consistency within a theme is achieved via a mechanism of references, which enables properties such as colors and fonts in individual controls to reference a common repository accessible to all other sections of the theme, which is called the "Base Theme Properties". Properties in that repository are not directly applied to controls' properties, but they can be referenced by the theme author when specifying the actual properties used by the controls. Additionally, a theme includes a section that is used by C1ThemeController to apply the theme to standard WinForms forms and controls such as buttons and labels.