From the signature editor, you can insert a border in your signature.
Select the block for which you want to insert a border, then expand the "Appearance" menu. From this menu, you can choose the type, color and thickness of borders.
Thanks to the "Padlock" pictogram, you can proportionally manage the thickness of the borders. Just click on the pictogram.
If the pictogram is white on a red background then by modifying the thickness on the left all the others will be modified proportionally.
If the padlock is white on a gray background then you can independently manage the thickness of each part of the block.