Two bugs:
1. [data-theme="dark"] block was defined BEFORE :root in source CSS,
so :root cascade overrode it. Moved to AFTER :root and bumped
specificity to html[data-theme="dark"] to beat plain :root.
2. Theme toggle button only existed in the topbar, which is hidden
on landing/login/sso-callback scenes. Added a ThemeToggle to
the Landing header so users can flip theme from the home page.
Confidence: high
Scope-risk: narrow
