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).
Placeholders are 1440x900 'Screenshot pending' PNGs that will be
overwritten by real captures via 'npm run screenshots' once a demo
user is provisioned.
