highlighting the current page in a list of links using css html

Question

I am trying to amend a friend's website which has been built so that there is a sub menu which appears on every page but is coded only once.

I want to be able to add a highlight to the link for the page you are currently viewing, but I have to do this all in one HTML snippet - so the code below precedes the HTML list which is used for navigation.

The list renders fine, except that I can't get the current page to highlight (the 'active' tag only highlights as you click the page).

I have read some other posts about adding 'current link' formatting in a separate file but, unfortunately, I have to include all the code in this snippet.

Given that, is what I am trying to achieve possible?