Here is an article on changing the existing css in Sitefinity: Applying CSS
We have plans to provide explanations for the common CSS files used in Sitefinity administration. Until then, however, you are more than welcome to contact us with any particular questions you have.
Here are some hints:
Administrative Part: go to ~/[your_project]/Sitefinity/Admin/Themes/Default/ . You could find the styles for the administration in there.
Public Part: in order to apply styles to the public controls, go to ~/[your_project]/Sitefinity/ControlTemplates/[name_of_module]/ and you could find the CSS file applied to the controls of the current module. For example, the public controls of the Libraries module are styled in this file: ~/Sitefinity/ControlTemplates/Libraries/librariesCommonLayout.css
Still, our recommendation is to make changes to the styles within the theme applied. First copy the an existing theme and rename it. Then, make the necessary changes to customize the appearance.
Here are some forum posts you may also find useful:
Hope this helps a bit. Let us know if we could assist you any further.
the Telerik team
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