@tuanphan Hello!
Like the others who have posted here, I too am looking to create a new button in the header of my site. The url is www.digilangua.net.
I have posted the code that you have provided and have attempted to create a button labeled "Teacher Login" in the header to which to attach an external link.
I tried adding the code you provided in the CSS space, but nothing happens, giving me the idea that I am the one not making it work correctly. Do you have any advice, please?