Source: dist/css/library_boot.css, line 6540
The toggles for bootstrap collapses use a + icon that twists to an x when the elements doesn't have collapsed or show as a class. to reduce confusion the class has a set rotation and speed attached to it. a:rotation is applied to the tow specific icons fa-times and fa-chevron-up only.
collapsed state.when the associated content is expanded the toggle will look like this
when the associated content is collapsed the class is added to the toggle and it will look like this.
<div class="flex_none p_2 [modifier]" aria-expanded="false" data-bs-toggle="collapse" data-bs-target="#pageHubChild" aria-controls="#pageHubChild">
<i class="a:rotation c_primary fa-fw fa-times fas self_center text_center"></i>
</div>
<div class="flex_none p_2 [modifier]" aria-expanded="false" data-bs-toggle="collapse" data-bs-target="#pageHubChild" aria-controls="#pageHubChild">
<i class="a:rotation c_primary fa-fw fa-chevron-up fas self_center text_center"></i>
</div>
<div class="flex_none p_2 [modifier]" aria-expanded="false" data-bs-toggle="collapse" data-bs-target="#pageHubChild" aria-controls="#pageHubChild">
<span class="fa-stack fa-2x">
<i class="fas fa-minus fa-stack-1x"></i>
<i class="fas fa-minus rotate_90 a:rotation fa-stack-1x "></i>
</span>
</div>
<div class="flex_none p_2 [modifier]" aria-expanded="false" data-bs-toggle="collapse" data-bs-target="#pageHubChild" aria-controls="#pageHubChild">
<span class="fa-stack fa-2x">
<i class="fas fa-square fa-stack-2x c_black"></i>
<i class="fas fa-minus fa-stack-1x c_white"></i>
<i class="fas fa-minus rotate_90 a:rotation fa-stack-1x c_white"></i>
</span>
</div>