Hi Bao-Nhi Vu,
Thank you for posting in this community.
In General, we can customize the Headers using the Text web part. And based on my test, it is available on a Team subsite homepage. And the below picture shows my test result.
And for more information, please refer to this article: Add text and tables to your page with the Text web part
Otherwise, if your idea is to customize the left-hand menu on the team subsite, please refer to this article: Customize the navigation on your SharePoint site
If your issue persists, could you please share a screenshot of this team subsite homepage and another page with the headers for a better understanding?
Best Regards,
May