The red hover is a little cheat to get around the lack of CSS for non-premium users. Unfortunately, there isn't much you can do other than btn-outline-info to another bootstrap color.

This is actually an older code and I recently found a better way of making the hover look nice. Change this line:

<a href="CHARACTER_LINK" class="btn btn-outline-info mx-auto w-100 rounded-0 tooltipster" title="CHARACTER_NAME" style="height: 150px; border: 0; opacity: 0.4; background-blend-mode:screen"

To say this:

a href="CHARACTER_LINK" class="btn btn-outline-warning mx-auto w-100 rounded-0 tooltipster" title="CHARACTER_NAME" style="height: 150px; border: 0; opacity: 0.4; background-blend-mode:luminosity"