This is in the documentation.
Arrows
Two sets of arrows are included with the plugin. Also included is a PhotoShop .psd file that can be used as a template to create custom arrows. Be sure to back-up any modified files, as they will be overwritten when the plugin is updated. To make update-proof modifications to the arrows, place the new arrow images in the same folder as the theme’s style.css. Then, add the following to your theme’s style.css:
.collapseomatic {
background-image: url(your_down_arrow_image_here.png) !important;
}
.colomat-close {
background-image: url(iour_up_arrow_image_here.png) !important;
}
The arrow postion can be set to display right of the trigger assigning the trigclass attribute a value of arrowright. The arrow can be completely removed by assigning the the trigclass a value of noarrow. See the Trigclass Attribute section below for more information.
https://plugins.twinpictures.de/plugins/collapse-o-matic/documentation/
