Since a better addon "Stylus" exists, this legacy "Stylish" version is only giving away your complete browsing history to a commercial and dubious 3rd party in exchange for sweets.
At first sight, it might conform to Mozilla's "no surprises" policy as the little cartoon doggie clearly begs for all urls. But it's not necessary, and the comfy link to userstyles is obviously just for grabbing your privcate data. Proof ist that there seems to be no way to manually check only for sites you actually would like a style for.
A lot of users will still use it because it's a well-known name and has been reviewed a lot in the good ol' days. Mozilla has a real problem with addons changing their policy for the worse. Avoid at all costs and use the alternative addon Stylus.