There is 100% backwards compatibility, because Reaper has to work even when naked. Therefore, every theme ever made remains feature complete to this very day. Some post-theme-design features will get translated or re-interpreted, and HiDPI support is theme dependent, but if you haven't noticed any of that bothering you then there is no down side to staying as you are.
If you don't know what you're after, I would encourage you to try out my
WT Theme Assembler Script, which is like a single theme that can cover a
lot of styles.