CSS:
1.Applies to all HTML Controls
2.Is applied on the Client Side in the Browser
3.We can apply multiple style sheets to a single page
4.The CSS supports cascading
5.The CSS cannot override the property values defined for a control.
6.Cannot be applied through the configuration files
7.Can be used directly via a reference to the css file location
8.Do not require any other resource like Skin files
9.In case of CSS you can define only style properties
Themes:
1.Applies to all the server controls
2.Is applied on the server rather than in the browser
3.But we cannot apply multiple themes to a single page. Only one theme we can apply for a single page.
4.But themes does not support cascading
5.But any property values defined in a theme, the theme property overrides the property values declaratively set on a control, unless we explicitly apply by using the StyleSheetTheme property.
6.Can be applied through Configuration Files.
7.All theme and Skin files should be placed in a special Asp.net folder called the “App_Themes” in order for the themes to work and behave normally.
8.Each theme should be associated with at least one Skin file.
9.But a theme can define multiple properties of a control not just style properties such as we can specify the graphics property for a control, template layout of a GridView control etc.