I came across this which says its currently not possible (https://adobespark.zendesk.com/hc/en-us/articles/1260801724449-Can-I-publish-Pages-with-Shared-Brands-)

Sharing brands for pages seems like an essential feature for my team. Is there any plan to add this ability? We need to have one person be in control of setting the brand for our team, then others can make and publish pages within our branding.