MSDN says
To Start with
Quote:
Themes are made up of a set of elements: skins, cascading style sheets (CSS), images, and other resources. At a minimum, a theme will contain skins
The below will override the individual styles
Quote:
If you set a page's Theme property, control settings in the theme and the page are merged to form the final settings for the control.If a control setting is defined in both the control and the theme, the control settings from the theme override any page settings on the control. This strategy enables the theme to create a consistent look across pages, even if controls on the pages already have individual property settings. For example, it allows you to apply a theme to a page you created in an earlier version of ASP.NET.
The below way it wont override
Quote:
Alternatively, you can apply a theme as a style sheet theme by setting the page's StyleSheetTheme property. In this case, local page settings take precedence over those defined in the theme when the setting is defined in both places. This is the model used by cascading style sheets. You might apply a theme as a style sheet theme if you want to be able to set the properties of individual controls on the page while still applying a theme for an overall look.
How to use Theme property
http://msdn.microsoft.com/en-us/library/tx35bd89(v=vs.85).aspx[
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Hope this helps to clear your doubt.