Anyone know which files to modify the blue bolt effect from Bowling bBash?
Best guess is this file: data\texture\effect\bowling.str
Caveat being that all 3 .bmps used in that .str are also used in about 13~14 other effects. So if you go in and change those 3 root .bmps, it will affect all other effects that also use them.
You could learn to edit the .str itself and redirect it to new .bmps though, worth a try if you're already thinking of playing around with client-side stuff like this.
Thread on that here (https://rathena.org/board/topic/130296-a-more-friendly-str-editor/).