In Chrome, right-click a comment bubble and select inspect element from the pop-up menu. The DevTools console will open up at the bottom of the browser. On the left pane will be a representation of the web elements, with the container for the bubble highlighted. On the right is the pane which shows all of the CSS rules which affect that element.
Since you are having a problem when the bubble is hovered, you need to force that state on so you can see the CSS rules for the hover state. Right-click the code line on the left pane which corresponds to the bubble and from the pop-up menu, select Force element state > :hover. This should make the bubble act like the mouse is being hovered over it.
Look at the CSS rules on the right pane for a rule which has the :hover pseudo element as part of the selector, and which has border properties set. This is the rule you will probably need to override.
Having said all of that, it could be just a matter of background positioning of the image file instead of it being a CSS border property issue. That is, for this type of animation where an element changes color on a mouse hover, what is commonly done is there is a large image file which contains a number of different icons in it. In this image file will be a white bubble and an orange bubble, for example. In the normal state, the background-image property for the bubble’s CSS will point to the image file, and the background-position property will point to the offset within the image file where the icon is located. The CSS rule will also have height & width properties that define the size of the icon. Then the CSS hover rule will change the background-position property of the element so it points to the orange balloon inside of the same file. It’s possible that the value of the background-position for the hover rule is slightly off, so that orange from an adjacent image is bleeding over. Or maybe the height and/or width properties for the hover rule are somehow bigger by a pixel or two, so again you’re getting some of the adjacent image bleeding over.