The border-bottom-width property sets the bottom border width of an element. Note: the Border Style property must be set for any of the other border properties to work.
e.g. '15' would set the bottom border for an element to 15 pixels (Microthemer automatically adds the 'px' unit if a unit isn't specified). Other commonly used units include 'em' and '%'. So you could enter '5%' in the Bottom Border Width field.