The options that you can use are
1. Put your stylesheet in ~/App_Data/Sitefinity/WebsiteTemplates/DefaultTemplate/App_Themes\THEME NAME/Global/ . The css put in Global folder will be automatically loaded. If the css is outside of this folder you have to reference it manually
2. Write your css inline in master page or control
3. If you use RadControls you can add skin file or make a global reference to a skin through your web.config.
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