Possible to add a tagline under site title in 7.1?

Hi Folks - Is it possible to add a text tagline under my text site title (both centred) in 7.1? 

Add to Home > Design > Custom CSS

a#site-title:after {
    content: "tuan phan";
    display: block;
}

I added the code:

a#site-title:after {
    content: "tuan phan";
    display: block;
}

but it makes the tagline appear directly on top the text site title (its a text title not an image) - see 1st screenshot....

I need a tagline below the site title - see 2nd screenshot...

Any suggestions?   Thanks!  

This should do the trick:

.header-title-text a::after {
  content: 'Tagline';
  display: block;
  font-size: 14px;
  line-height: 1em;
}

Thanks for replying - I used your code which sorted the Site Title & Tagline in desktop view but the navigation is aligned to tagline which looks wrong. In Mobile view the tagline is same size as Title....

Cheeky to ask but is there anyway to align the navigation with the Title not the Tagline and sort the mobile view by reducing the tagline size? I'm guessing its because it's 7.1? Maybe I should use graphic instead.... 

See screenshots of desktop and mobile views....   

Thank you!

I'd probably use a graphic.