Helper class to change theme resources at runtime

ThemeHelper.cs

using Telerik.XamarinForms.Common;
using Xamarin.Forms;

namespace YourAwesomeApp.Helpers
{
    public static class ThemeHelper
    {
        public static void ChangeTheme(string themeName)
        {
            if(Application.Current.Resources != null)
                Application.Current.Resources.Clear();

            var resourceDictionary = new RadResourceDictionary();

            // Select your chose theme's colors
            if (themeName == "Default")
            {
                // Don't add a separate resource dictionary to use the Default theme
            }
            else if (themeName == "TelerikBlue")
            {
                // This is the BlueTheme delivered with UI for Xamarin
                resourceDictionary.MergedDictionaries.Add(new BlueResources());
            }
            else if (themeName == "Custom")
            {
                // This is the custom ResourceDictionary you've created
                resourceDictionary.MergedDictionaries.Add(new CustomTheme());
            }
            
            // Merge the rest of the resources
            resourceDictionary.MergedDictionaries.Add(new Telerik.XamarinForms.Input.TelerikThemeStyles());
            resourceDictionary.MergedDictionaries.Add(new Telerik.XamarinForms.Primitives.TelerikThemeStyles());
            resourceDictionary.MergedDictionaries.Add(new Telerik.XamarinForms.Chart.TelerikThemeStyles());
            resourceDictionary.MergedDictionaries.Add(new Telerik.XamarinForms.DataControls.TelerikThemeStyles());
            resourceDictionary.MergedDictionaries.Add(new Telerik.XamarinForms.DataGrid.TelerikThemeStyles());

            Application.Current.Resources = resourceDictionary;
        }
    }
}