There are many people who’ve created themes to make their Discourse forum a bit “more like phpBB” to make the transition to Discourse a smaller step for the members of their community.
But in the end it’s just an attempt of postponing the inevitable, and in most communities we see people asking for a more modern look and feel within a few days, weeks or months. And for those “older people” a period of small changes might be much worse than one big change. So my advice is to just bite the bullet and dive in.