Added --nav-text and --nav-text-muted CSS variables. All nav
selectors (.top-nav, .nav-brand, .nav-link, .nav-btn, .nav-stats)
use these instead of --text/--text-muted. Nav Text is in Basic
settings. Nav Muted Text in Advanced.
This is separate from page text because nav sits on a dark
background — page text color would be unreadable on the nav.