ASP.NET web forms change bootstrap theme dynamicly - css

Question

my main problem is that I want to change the bootstrap theme of my Website with a Dropdown list of themes and a button.
#model IEnumerable<ProjectManagementTool.ServiceReference1.Theme>
#{
ViewBag.Title = "Settings";
Layout = "~/Views/Shared/_Layout.cshtml";
}
<form action="/Home/Settings" method="get">
<h2>Settings</h2>
<br />
<h4>Change Theme</h4>
#Html.DropDownList("GetThemes", null, htmlAttributes: new { #class = "form-control" })
<br />
<p>
<input class="btn btn-default" type="submit" value="Save"/>
</p>
#foreach (var item in Model)
{
#Html.DisplayFor(modelItem => item.Path)
<br/>
}
</form>
The foreach loop is just to show the diffrent paths.
I have a database aswell with a themes table:
Now the question is, how can I change the bootstrap?
I know that there is this code tho change the theme.
#Styles.Render("~/Content/flatly.bootstrap.css")
But it should be in the _layout.cshtml file to set it for the whole website.
The background is working with a service but this is not important here.
Screenshot of the website:

Answer 1

You could write an action that returns the path of the css file to be used depending on the user settings.
Something along the lines of
#Styles.Render(Html.Action("UserCssPath", "Settings").ToString());
Because you don't have access to a ViewModel in your layout page, the controller must use another mechanism to find out for which user the action was invoked. For example, we store a User Context object with the required data in the HttpContext.Current.Session. This could be written once on logon. The UserCssPath action can then access the DB to find the correct CSS path and return it: return Content(cssPath);