Customizing Intelligent Menu Script

Okay, I’ve been at this for a few hours now with no luck (although I imagine the answer is simple…)

I just placed a new nav bar in my header ( https://www.withinwithout.org ) and I’d like to use Matt’s Intelligent Menu Script (https://photomatt.net/scripts/intellimenu) to highlight the current page with a different background-color (#A37B45) in the button. I’ve tried and tried and only succeed in stacking all the buttons on top of each other and breaking the template….

I know it’s likely just matter of me not fully understanding CSS and PHP, but perhaps one of you lovely and kind experts would like to point me in the right direction… I seem to have tried all the things I can think of. ??