Site Title in Header, two different font sizes possible?

I am using Thanks Roy, and I want to have (in the header) a big title and a longer subtitle in a smaller font size underneath.
where can I do this?
In Settings I can only insert a title and have no option to choose size or a line break.
In Appearance /configure Theme is also no possibility…

would this change be in CSS Editor, if so, what would be the necessary code?

As I tried to help myself by adding the subtitel to the picture, which of course looks terrible because the text does not get sharp with the solution of the picture …

I kindly ask again : can anyone help me with this? It would be much appreciated!

It’s going to be tough to do this any way but just editing the theme: you can’t put markup in your title so there’s nothing to grab and style differently.