PacketFence - Captive Portal styling

[googlemad]

We are currently implementing NHS WiFi using PacketFence with the captive portal for guest user registration.

 

I've been sent across some guidelines which basically state the captive portal should resemble the image shown on the following page as closely as possible (about halfway down the page) :

 

https://digital.nhs.uk/nhs-wi-fi/how-to-set-up-nhs-wi-fi-in-GP-practices

 

The trouble is I'm rubbish with HTML and CSS and although I can see how you can modify bits of the HTML files within PacketFence I wouldn't know where to begin! Anyone done something similar before in terms of styling the captive portal?