At #8c4a3c the highlight blended into the lighter board in the light theme. Give the light theme a lighter burgundy #9c5849 while the dark theme keeps #8c4a3c — the two are tuned per theme because perceived contrast depends on the surrounding board tone.
The gold/brown recent colour shared the tile's warm hue, so it could not separate from both the near-black glyph (when dark) and the tan tile (when light) at once. Switch --tile-recent to a burgundy #8c4a3c whose red hue stays distinct from both, in light and dark, and unify the value across all three theme blocks.
Dark theme: the 2x/3x bonus-square pairs were too close to tell apart. Soften the 2x squares (sky blue #4a779b, rose #a8636b) and deepen the 3x squares (#2c527a, #9c3f34) so each pair reads as two distinct steps. Light theme is unchanged.
Last-word highlight (both themes): stop tinting the tile background — the tile keeps its normal fill, and instead the placed letters (not the point values) are drawn in the recent-move colour. The opponent-just-moved flash now pulses the letter between its normal colour and the recent colour, with no background animation and no white peak.
Reconcile the explicit [data-theme=dark] --tile-recent with the OS-dark value so the highlight reads the same however dark is selected, and darken --tile-recent a step in every theme. Update docs/UI_DESIGN.md.
Ilia Denisov