Use American English spelling throughout

web/index.html

@@ -9,7 +9,7 @@
       theme, which a media query cannot do — it only knows what the OS prefers,
       not what the user picked here. There used to be two, both with media
       attributes, which meant the selector in applyTheme (:not([media])) matched
-      neither and the colour never moved off whatever the OS implied.
+      neither and the color never moved off whatever the OS implied.
 
       The initial value is the default theme's background, so the browser chrome
       is right from the first paint rather than only once JS has run.