I have a very strange error with https://developer.salesforce.com/trailhead/lex_dev_lc_basics/lex_dev_lc_basics_create. I have added a style to my campingHeader component:

.THIS {
}

h1.THIS {
   font-size:18px;  
}

but trailhead returns an error:
Challenge Not yet complete... here's what's wrong:
The component is not using the correct font size.

Anybody can help?