Theme "replacements/variables" in 4.x

Just wondering what the best way to create a "skin replacement" is? Eg, this code is used in 3.4.x to output a topic icon, but custom ones can be created using the "Manage Replacements" feature, which I've used quite a lot for my current themes.

{parse replacement="t_unread"}

What's the best way to replicate this feature in 4.x? Would a theme setting be the best, or is there a way to create a variable which can be used in all template files?

Thanks!