There’s probably another workable way.
Here is a link to another user who installed and uninstalled oh-my-fish, the framework of those (messy) themes.
There is a command to remove everything, or going to the reported link, instructions to install and remove the single themes, which you might consider. Otherwise if you uninstall the framework maybe might have something remaining from the themes…
In addition to that, as suggested in the other thread, why don’t you simply try resetting the Fish config with the relevant flag in the Garuda Assistant?