Some railways have a theme, others don't, but those that do are generally pretty consistent. The smaller ones generally cannot stretch to such a luxury, and run whatever they can muster to provide revenue. It is deceptively expensive (in volunteers' time as well as money) to maintain rolling stock, let alone repaint it. Most preserved railways, understandably, focus on steam where they can, as that is the real pull for non-enthusiasts as well as enthusiasts. Some may be able to lay on galas where modern-liveried diesels feature but matching coaches are very rarely available. In decades to come, some preserved lines will feature more modern scenes (after all Mk1s and older will only keep going for so long) but, for now, 1940s-1960s liveries are going to remain dominant I think. Main line steam rakes are more and more featuring post-Mk1 stock and that can surely only cascade to preserved lines.