How to change the default theme at code behind not through Admin section?

What is the method in controller that is responsible for the default theme?
I want to load different theme for each Affiliate in my current webstore.
How to do that?

arlen_bs wrote:

if (_workContext.CurrentCustomer.AffiliateId == 1)
            {
                currentTheme = _themeProvider.GetThemeConfiguration("LightBlue");
            }
else
{

    currentTheme = _themeProvider.GetThemeConfiguration(_themeContext.WorkingDesktopTheme);
}