The reading highlight had exactly one look: an accent-colored pill on the word being spoken. Before I commit to one signature style, I wanted to try several, so I made the highlight itself swappable. The current-word treatment is now a setting you pick from the transport bar, saved like your theme, and it changes how the spoken word is marked without touching any of the timing machinery underneath. Each word is already its own small view, so a new effect just attaches in place; there's no second overlay to keep in sync with the text.
Then I built a menu to choose from. Tint is the quietest: keep the sentence band and just color the current word, no pill at all. Karaoke fills the word with the accent left to right as it's spoken, like a progress bar inside the letters, then fades back to ink once it's read. Shimmer runs a bright sheen across the word, and Wave gives it a tiny travelling ripple. The last two are Metal shaders.
What I cared about under all of it was that none of this should cost frame rate. The fills and the shimmer run off a sub-word progress value the playback driver already computes each tick (rate-independent, so it's honest at any speed), and only the current word redraws each frame, never the page. The Metal shaders are the stitchable kind that compile into the package itself, so they build the same under plain swift build and Xcode and the fast per-package test loop stays green. The point wasn't to ship five highlights. It was to feel each one in a real book before deciding which one is Aoede's.