Qt's SVG renderer has no CSS cascade, so a `currentColor` fill has to be
substituted before the bytes reach QSvgRenderer.
- Match and replace on bytes: both the sentinel and the replacement are
ASCII, so the document never goes through a UTF-16 round trip, and files
that are not valid UTF-8 survive unchanged.
- Document that the match is a plain case-insensitive substring, so an
identifier that merely starts with the sentinel gets rewritten too.
