I started off Wordpressing...then I discovered Moodle and made my first Moodle (1.9.7) theme based on a Wordpress theme (Japan style). It was a steep learning curve!
OK...back to you custom Block.
I did not know about the 'section link' block...so I have just been playing with it. The highlight in the php $highlight refers to the course topic section or weekly section, in the center of the course page. If you turn editing ON you get to see a light bulb, which, if you click it, 'highlights' that section. This is what Joseph was refering to in this earlier discussion.
So if you had Section 3 highlighted in that course page. When you look at the Section Links block it has all the section numbers listed (topic sections are numbered). Under the numbers it says, "Jump to current section*. This 'current section' is the one which is highlighted. So clicking on 'Jump to current section' moves Section 3 to the top of the page. A bit like you get with Bookmarks on a web page.
So really there is nothing for you to 'highlight' in your block unless you have added something. Also...blocks don't have stylesheets so I guess they are just funcional objects on a page and rely on the theme to style them.
Hope this makes sense?
Cheers
Mary