Borders get too bright with very dark custom background colors #16114

Hey, I've been using Dark Reader with custom colors synced from my desktop theme and noticed that on very dark backgrounds the borders on light sites stand out way too much, almost like highlighted rows. With a mid-dark background they look fine.

Example on GitHub's file list (its borders are #d1d9e0):

• background #2d353b → border #364047, contrast 1.18, looks right
• background #001419 → border #00647d, contrast 2.80, looks like every row is highlighted

I dug into it and it comes from modifyBorderHSL in src/inject/dynamic-theme/modify-colors.ts. The lightness there is absolute:

const lx = scale(l, 0, 1, 0.5, 0.2);

so borders always end up around 0.2–0.5 lightness no matter how dark the background is. On top of that, light neutral borders take the background's hue and saturation, which is why a saturated dark background turns them cyan.

I tried making it relative to the background and it fixed it for me:

const lx = Math.min(1, Math.max(0, scale(l, 0, 1, 0.5, 0.2) + (poleBg.l - 0.21)));

0.21 is roughly where the current output already looks good, so mid-dark backgrounds barely change, and darker ones get proportionally darker borders. With that patch, the same GitHub borders come out at contrast ~1.14 on #001419, 1.15 on #2d353b, 1.09 on Tokyo Night #1a1b26 and 1.05 on pure black, and the page background stays exactly the same.

I'm running it locally in Zen now with no issues. Happy to open a PR if you think this is the right direction, or adjust it however you prefer.