Since someone already mentioned thickening lines if the line art is on a white background, I’ll tell you about the other solution that works in that situation. Go to the filter tab and run the G’MIC filter. When it opens go down the category list to Artistic and open that, then search down the list to Poster Edges. When that is opened you’ll have a whole bunch of sliders that allow you to antialias, smooth and thicken the line work. The G’MIC filter is a plugin that is a part of Krita but you have to install it first, I think in settings or configure Krita. Anyway the G’Mic filter has a ton of things to play with and is worth installing.