The optional thickness attribute is the line thickness in pixels.
Source
<xs:complexType name="LineStyleType"><xs:annotation><xs:documentation>The LineStyleType defines a line style.</xs:documentation></xs:annotation><xs:simpleContent><xs:extension base="LineStyleEnumType"><xs:attribute name="thickness" type="xs:positiveInteger" default="1"><xs:annotation><xs:documentation>The optional thickness attribute is the line thickness in pixels.</xs:documentation></xs:annotation></xs:attribute></xs:extension></xs:simpleContent></xs:complexType>
<xs:attribute name="thickness" type="xs:positiveInteger" default="1"><xs:annotation><xs:documentation>The optional thickness attribute is the line thickness in pixels.</xs:documentation></xs:annotation></xs:attribute>