We are using Multisite for our Franchise Microsites.

We are using Multisite for our Franchise Microsites. Example here Securis.me.

My question is one of programming - We want all of the sites to use the exact same theme. However we want the phone number at in the header and footer file to be changed for each site. I'm pretty sure we need our php guy to build us an option page withing the theme or create a custom plugin where we can enter the information entered for each site once the site is created. This way each site would use the same theme but have different contact details. Is this the correct approach to this? I'm not a wordpress expert at all but doing some reading it seems that creating an option page is pretty easy.