@sujie. You have to realize that I don’t have a link to your page. So I can’t examine your css. You may have css in your template that could be overriding what I just showed you. Maybe if you provide a link to your webpage, I would be able to properly assist you.
EIther way, please note that it would be good for you to learn how to look at the DOM elements CSS on your webpages. Just right click on the heading and select “Inspect Element”, and you will see how your styles are being applied. Learning CSS is important to being an effective webmaster.