CustomThemeSelector.cs

using Orchard.Themes;

namespace Something.Themes {
    public class CustomThemeSelector : IThemeSelector {

        public ThemeSelectorResult GetTheme(RequestContext context) {

            if (someCondition) {
                  return new ThemeSelectorResult {
                      Priority = 10,
                      ThemeName = selectedTheme
                  };
            }

            return null;
        }
    }
}