i do not know from where this css is coming
I'm assuming your asking where are your CSS files located? It is wherever you put them.
To be sure, go into your MVC project, look for App_Start folder, find BundleConfig.cs and there you should see the location of all javascript and CSS files and their referenced locations.
For example, this is one of the bundles i have in my MVC project for CSS files
bundles.Add(new StyleBundle("~/Content/themes/base/css").Include("~/Content/themes/base/jquery.ui.core.css",
"~/Content/themes/base/jquery.ui.resizable.css",
"~/Content/themes/base/jquery.ui.selectable.css",
"~/Content/themes/base/jquery.ui.accordion.css",
"~/Content/themes/base/jquery.ui.autocomplete.css",
"~/Content/themes/base/jquery.ui.button.css",
"~/Content/themes/base/jquery.ui.dialog.css",
"~/Content/themes/base/jquery.ui.slider.css",
"~/Content/themes/base/jquery.ui.tabs.css",
"~/Content/themes/base/jquery.ui.datepicker.css",
"~/Content/themes/base/jquery.ui.progressbar.css",
"~/Content/themes/base/jquery.ui.theme.css"));
So the location of my CSS for those files are, starting from my MVC project, in the Content/themes/base/ folder.
If you need to know, from the browser, where the CSS is for a specific element, either learn how to use inspect element (simple google search of "how to use inspect element in <insert your browser name here") or do a simple search for your CSS element in your entire project.