[fix] simple theme: don't crash when the checker is enabled

The macro "checkbox" in macros.html uses the macro "icon_small"
from icons.html

The commit imports icon_small in macros.html to fix the issue.
It works because the macros in macros.html are imported with the Jinja2 context.

See https://jinja.palletsprojects.com/en/3.0.x/templates/#import-visibility

close #819