The default Lnk.Bio layout and most Themes come with a border around the Lnks to highlight their content. The border looks something like this:

If you wish to remove such border, you can do so very quickly from Style, scroll down until you see your Lnk Block, and click on Change Effect as shown below:

Once the tab is open, you will see the No Border button effect. Select it, as shown below:

All done! All your Lnks will now appear without any border. Here's an example: