You are right in describing what the desired result is. And you can
get it using the method I gave you.
It looks as if you are omitting the first step 'Just highlight the
span element'.
I was assuming that you knew how to highlight an element. Maybe you
don't so here’s how.
1 - Click anywhere in the element i.e in the word 'dark'.
2 - Either (a) below the main window in the area which as yet seems to
have no name you will see <body> <p> <span> (there may be other text
between <body> and <p>), Click <span>, you should see the word 'dark'
highlighted. It is now selected.
or (b) Use the DOM Explorer. You should see the span highlighted (i.e.
over a grey background). Click it. You should see the background
colour change in the DOM Explorer (not in the main window). The
element is selected and you can manipulate it.