What the paper variants change, and what they leave alone.
Light and dark are neutral. The two paper variants take the same colors and set them on a warmer page. This note is about what that does and does not mean.
A surface treatment
Paper changes two things: the surfaces, and the mono inks that sit on them. The canvas of Light Paper is #f4f2ec, a low-chroma warm white. The canvas of Dark Paper is #111113, a near-black with a faint cool lean. Text, comments and punctuation take inks from a separate paper ramp that shares that hue.
Everything chromatic stays where it was. Green, blue and orange keep the coordinates of the standard variant, so a keyword is the same color in Light and in Light Paper. Paper does not warm every color, and it should not.
Why not simply tint the neutrals
The neutral scale is exactly neutral, with chroma zero. It is the most honest gray, and it is the reference the accent colors were tuned against. Paper is a second ramp with a hue of about 91.48° and a chroma of about 0.008, a little less toward the extremes. It stays quiet on purpose: just enough to read as paper, not enough to read as sepia.
What it does not claim
A warm page can feel gentler. It is not evidence of less eye strain, and LiG does not say so. Contrast is the part that can be measured. Against its canvas, Light Paper's primary ink reads about 5.87:1 and Dark Paper's about 9.41:1; the muted inks are lower on purpose, because muted should look muted.
If you work in a bright room, try both and keep the one you stop noticing. That is the only test that counts.