Enable dark theme if user prefers dark color scheme - no javascript version

[AntonKhorev]

We probably can enable dark mode now.

This version uses system settings with media queries and doesn't need javascript. It is enabled by the Bootstrap option $color-mode-type: media-query. Previous version #4653 used javascript to track system settings and change data-bs-theme attributes accordingly.

The problem with this no-javascript approach is that data-bs-theme attributes are ignored. But we're already using them for inverting close buttons on dark banners. When in light mode, dark banners have their close buttons inverted because banners themselves have a dark theme attribute. Aside from not working with $color-mode-type: media-query, this approach has another drawback: you can't invert buttons on light banners in dark mode. Once the entire webpage gets its dark mode attribute, you can't go back to light mode.

The solution in this PR is:

• use $color-mode-type: media-query that will take care of most of light/dark mode styling
• use data-bs-theme on banners
• add css to enable/disable close button inversion inside elements with data-bs-theme

[AntonKhorev]

Was conflicting with scale control color fixes, now updated.