Introduced in v5.2.4 It is possible to define the toc-open-icon and toc-closed-icon variable, to change the icons used by the toc macros. This setting works for all toc-macro variants
<$let toc-open-icon="$:/core/images/fold-button" toc-closed-icon="$:/core/images/folder">
<div class="tc-table-of-contents">
<<toc-selective-expandable "Contents">>
</div>
</$let>