darker lines:
colors > brightness-contrast > contrast = 40
filters > generic > erode
filters > artistic > cartoon > mask radius = 50 / percent black = 0.7
filters > artistic > oilify > mask radius = 1

thicker lines:
colors > brightness-contrast > contrast = 40
select by color tool > click white background > ctrl+i
select > grow > amount = 4
pencil tool > maximum size > draw over lines