public class SendEmail {
public static void sendOpportunityEmails() {

List<Messaging.SingleEmailMessage> mailList = new List<Messaging.SingleEmailMessage>();

    // Fetch Opportunity with Owner Email
    List<Opportunity> oppList = [SELECT Id, Name, Owner.Email FROM Opportunity WHERE Id = '006WU000009lzEBYAY'];

    for (Opportunity opp : oppList) {
        if (opp.Owner.Email != null) { // Ensure Owner has an email
            Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
            email.setTargetObjectId(opp.OwnerId); // Use email address
            email.setSaveAsActivity(false);
            email.setSubject('Opportunity Update');
            //email.setPlainTextBody('Hello, an update is available for Opportunity: ' + opp.Name);
             email.setHtmlBody('<h1> Hello </h1> <br> <h2 style="color: green"> Radhe Radhe </h2> ');
            mailList.add(email);
        }
    }

    if (!mailList.isEmpty()) {
        Messaging.sendEmail(mailList);
    }
}

}