First make sure you link the correct files.
App_Start / BundleConfig.cs
// you want to remove this bundles.Add(new StyleBundle("~/Content/themes/base/css").Include( "~/Content/themes/base/jquery-ui.core.css", ... ); // and add this bundles.Add(new StyleBundle("~/Content/themes/ui-lightness/css").Include( "~/Content/themes/ui-lightness/jquery-ui.core.css", ... );
Then specify the set in your layout.
_Layout.cshtml
@Styles.Render("~/Content/themes/base/css") // remove this @Styles.Render("~/Content/themes/ui-lightness/css") // add this
All of this assumes that you installed the theme in /Content/themes
The easiest way is to find the base and replace it with ui-lightness in the layout and package configuration.
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