The link arrows are 6px * 6px elements marked with the following CSS classes:
scheduler_default_link_arrow_right
scheduler_default_link_arrow_left
scheduler_default_link_arrow_down
scheduler_default_link_arrow_up
The default styles (from the built-in theme) are as follows:
.scheduler_default_link_arrow_right:before { content: ''; border-width: 6px; border-color: transparent transparent transparent #cc0000; border-style: solid; width: 0px; height:0px; position: absolute; }
.scheduler_default_link_arrow_left:before { content: ''; border-width: 6px; border-color: transparent #cc0000 transparent transparent; border-style: solid; width: 0px; height:0px; position: absolute; }
.scheduler_default_link_arrow_down:before { content: ''; border-width: 6px; border-color: #cc0000 transparent transparent transparent; border-style: solid; width: 0px; height:0px; position: absolute; }
.scheduler_default_link_arrow_up:before { content: ''; border-width: 6px; border-color: transparent transparent #cc0000 transparent; border-style: solid; width: 0px; height:0px; position: absolute; }