A signature copied from Outlook in dark mode comes with its text lightened
and backgrounds darkened, each color marked !important: black text is
white, so on the light sheet a signature is shown on, and in recipients'
light-mode mail, most of its lines vanished.

Those colors get their lightness back with the same hue, inverting the
transform Outlook's editor uses (CIELAB lightness flipped into #333..white).
Only text that would get darker and backgrounds that would get lighter
change, so it can only make a signature easier to read on white. The Rust
side does it as a signature is cleaned, so it's kept and sent right; the
editor does it as one is pasted or loaded, so it shows right, and a
signature saved before this is fixed the next time it's opened.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>