What did you change in /css/styles.css
I had a .navbar-scrolled thing happening in that stylesheet that changed the navbar from having a transparent background to a #fff filled background on scroll, in exactly the same way as I have on this site:
https://moteviewholidaylets.co.uk/index.php
However, that is no longer working on this site:
https://www.thevaultartscentre.co.uk/index.php
....and I haven't made any changes.
Weirder still, I hadn't realised it was styles.css you had changed so I have overwritten that now with the stylesheet that the scrolled functionality worked with previously.
NJ