I don’t like the idea either. I looked through the files and found, where the style is set:
rt-prettyphoto.php
line 403:
font-family:<?php echo $royal_prettyphoto_font_family; ?>;
The easy way to solve the problem, is to just change this line to something like:
font-family: weblySL, Helvetica, sans-serif;
you could also try to get an other entry to the list of royal_prettyphoto_font_family.
If you also don’t want that the plugin gets the fonts from Google delete line 356:
@import url(https://fonts.googleapis.com/css?family=Autour+One|Meie+Script|Armata|Rum+Raisin|Sonsie+One|Kavoon|Denk+One|Gravitas+One|Racing+Sans+One|Nosifer|Ropa+Sans|Offside|Titan+One|Paytone+One|Audiowide|Righteous|Faster+One|Russo+One|Germania+One|Krona+One|Emblema+One|Creepster|Delius+Unicase|Wallpoet|Sansita+One|Monoton|Kenia|Monofett);
Cheers!