Couldn't there be some template-neutral images that can be set in pageparts, as opposed to having a different set of images for each template?
In the classic template, there's this:
style:classic
It makes the core use alternative style of collapsible handles.
The core can default to a certain set of buttons, while a flag like the one mentioned above would allow a template to insert its own buttons instead. Should be pretty straightforward to program, will make templates with their own buttons backward-compatible, and will prevent the confusion of whether the buttons are one set for all templates or a separate set for individual templates. Best of both worlds.