Hi @ChristineTombleson1 , there isn't any off-the-shelf functionality allowing us to customise the branding of the web maps that we make available to our field staff through the ArcGIS Field Maps mobile app.
The best you could do in this scenario is add the title "ShoreWatch" to the name of the web map and you could customise pop-ups in the web map to contain some branding as well. However the mobile app theme can't be customised.