[BS5.3] Replace .navbar-dark by data attribute and other style adjustments

[chalin]

• Contributes to Upgrade to Bootstrap 5.3 #1528
• Replaces .navbar-dark by data-bs-theme="dark" data attribute
• Adjusts search element styles to work with the new navbar's dark mode and in the left-nav too (not in dark mode)

Preview: https://deploy-preview-1896--docsydocs.netlify.app/docs/

Screenshots

Light mode (default):

Dark mode:

[fekete-robert]

When using a dark system theme under firefox the text in the navbar search is dark (tested on mac and linux):

Using Brave (a chrome derivative) it's fine on both systems.

But this is probably an edge case, since firefox has a marginal market share, so we can consider it a known issue and go forward.

[chalin]

Thanks for that careful cross-browser check. Let's track that issue via:

• [Dark mode] Text in the navbar search is dark under FireFox #1906