Clarify what a theme actually changes beyond the header accent

A theme's font_family applies to every text element in a modern-style
widget, not just the header title, and radius/shadow change the card's
whole look -- worth spelling out since the accent color alone
undersells how much a theme actually does, especially on text-heavy
widgets, and on the two widget kinds (battery, static/whiteboard) that
have no header at all.

docs/widgets.md

@@ -227,6 +227,17 @@ three-layer CSS custom-property theme system -- this is an original
 reimplementation of that *architecture*, not a copy of its token file
 (see this repo's `CLAUDE.md` on copyleft dependencies).
 
+**What a theme actually changes, in practice**: the header/accent color
+is the most visible change, but `font_family` applies to *every* text
+element in the widget, not just the header title -- day labels,
+temperatures, task rows, event times, day numbers all switch fonts too
+(e.g. "Moss" is serif, "Ochre" a slab serif), often more noticeable than
+the accent color on text-heavy widgets. `radius`/`shadow` change the
+card's corners and whether it has a drop shadow at all (e.g. "Slate" is
+square-cornered with no shadow). Battery and text/static/whiteboard have
+no header, so for those a theme only shows up as font/radius/shadow
+differences, not an accent color.
+
 **A theme is purely stylistic, never functional color-coding.** Battery's
 charge-level red/yellow/green, calendar/tasks' per-owner event color
 chips, and text's user-authored inline run colors are status/identity