How can I change the appearance of the displayed messages?

You can set the message content in the module.

To modify the appearance:

Copy the files from the directory:
/modules/pshowpricelog/views/templates/price_box/

To the directory:
/themes/*/modules/pshowpricelog/views/templates/price_box/

Then edit the .tpl files in:
/themes/*/modules/pshowpricelog/views/templates/price_box/