I don't seem to find an answer here, so I'll post my question.
I don't like the look of thumbnails, mostly because it doesn't match the look of the site: how can I show thumbnails without this fading effect in the borders?
Honestly I don't understand why you implemented this styling.
I believe a plugin should play with the front-end styles at all (unless when strictly necessary): this would avoid any style conflict with the active theme.
I'm afraid you're going to suggest to tweak the CSS, rather than adding an option in the plugin.