Use data-bs-theme='dark' and modernize dark styles (#42423)

Replace legacy 'navbar-dark' classes with the newer data-bs-theme='dark' attribute across example pages and adjust related dark-mode styling. Other updates: remove redundant 'btn-close-white', change 'text-muted' to 'text-body-secondary', replace 'border-right' with 'border-end' for logical edge utilities, and apply data-bs-theme to offcanvas/dropdown elements where appropriate. These changes standardize dark theme handling and update utilities in multiple site examples.