If the API doesn't offer a way to configure this, and it only returns a flat image (.png, .jpg, etc.) then you're pretty much stuck with standard image manipulation techniques, as far as I'm aware.

You'll need to develop them yourself in accordance with the effects you desire. Standard stuff like contrast and colour grading might not be too difficult.

Stuff like adjusting line thicknesses is likely to be surprisingly tricky; creating a filter that can distinguish, say, the line in a horizontal rule from the lines in text is not likely to be an easy task with a reliably good output.

This might be easier if your third-party API returns something like an SVG, which contains vector values amenable to manipulation.