Every now and then you just want to just add more to your SharePoint environment. Such as, for example, two custom CSS files. You can add the CSS files indeed to a master page. But that is actually not the way to do it. So make a copy of the master page that you want to use and then add the CSS file.
Adding custom CSS file
<head></head> <sharepoint:cssregistration id="CssRegistration01" name="Themable/oslo.css" runat="server" enablecsstheming="true"></sharepoint:cssregistration> <sharepoint:cssregistration id="CssRegistration02" name="/sites/nl/SiteAssets/StyleTopOfPage.css" runat="server" after="oslo.css"></sharepoint:cssregistration>
What is CSS?
CSS stands for Cascading Style Sheets
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