The default look of code typeset using the Latex package listings is one of the worst design choices ever. So many people keep this default of fixed width, too far spaced columns with a non-fixed-width font.

1. Do \lstset{columns=fullflexible}
2. Reduce intercolumn spacing
3. Optionally, use a nice-looking monospace font.

Also screen can show colors now, so consider using some nice contrasted colors like dark red here and there.

For comparison

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