Starlight's default uses --sl-color-text-accent (our teal #00d4aa) as
the active-item background, which gives ~3:1 contrast vs the dark
inverted text — under WCAG AA (4.5:1). Override with primary blue
+ white text for ~9:1.
Note: customCss loads before Starlight's component CSS, so the rule
needs (0,2,1) specificity to win against Starlight's scoped (0,2,0).
