BuddyPress.org

@giuseppe23 Could be border around item-list li . Another guess is to look for box-shadow styles around what I mentioned above or something else. Sorry, can only guess.

You can copy all content of your stylesheet and paste at pastebin.com, then post the generated URL here. Better chances there than all this guessing.

@giuseppe23 I can only guess at what can be done to remove the border based on image you posted alone. Check if you added borders to:
#buddypress ul#activity-stream or #buddypress ul.actitivity-list

@giuseppe23 The following should help you:
https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Forms/Styling_HTML_forms

Style Web Forms Using CSS

or Google “styling forms” e.g. to get so much more styling tips