Apply OOTB theme via CSOM

OOTBTheme.cs

string userName = "admin@tenant.onmicrosoft.com";
string password = "<your-password>";

string siteUrl = "https://<tenant>.sharepoint.com/sites/testsite/";

using (ClientContext clientContext = new ClientContext(siteUrl))
{
	SecureString securePassword = new SecureString();
	foreach (char c in password.ToCharArray())
	{
		securePassword.AppendChar(c);
	}

	clientContext.AuthenticationMode = ClientAuthenticationMode.Default;
	clientContext.Credentials = new SharePointOnlineCredentials(userName, securePassword);

	ThemeManager.ApplyTheme(clientContext.Web, OfficeDevPnP.Core.Enums.SharePointTheme.Orange);

}