Just a slight correction, the settings are actually saved by default to the browser's LocalStorage (using cookies as a fallback just in case), but like Plykiya said clearing local browser data will reset the local settings exactly the same.
Also, to clarify just in case, having the settings be local and specific to the browser you're using instead of being tied to your account is very much intended. If the same user is reading manga on both their desktop computer and smartphone, it's likely they wouldn't want to use the exact same settings for both.