Please note Purity III current style support Kunena 5.0.7 only. In the new update there is changes in the style this is why its not loading the style.
Or you can enable bootstrap CSS from kunena template and use its own style.
You can see the error is not from the template.
It’s from the extension, you can turn off the error report to NONE to hide the warning and report the error to extension support team so they have a look at it.