Use custom CSS style sheet in sharepoint 2013

Create Html of master page

Then convert into Design Manager.

after conversion html into master page


after conversion open html file 


Find following code
----
<!--MS:<SharePoint:CssRegistration Name="Themable/corev15.css" runat="server">-->
        <!--ME:</SharePoint:CssRegistration>-->

-----







Then Change URL
-----
        <!--MS:<SharePoint:CssRegistration Name="/_catalogs/masterpage/Gem_Ambulance/css/corev15_update.css" After="corev15.css" runat="server">-->
        <!--ME:</SharePoint:CssRegistration>-->
-----



one more thing you can use both in html like following Code


<!--MS:<SharePoint:CssRegistration Name="Themable/corev15.css" runat="server">-->
        <!--ME:</SharePoint:CssRegistration>-->
        <!--MS:<SharePoint:CssRegistration Name="/_catalogs/masterpage/Gem_Ambulance/css/corev15_update.css" After="corev15.css" runat="server">-->
        <!--ME:</SharePoint:CssRegistration>-->



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