Fix: make theme selector tick icon visible when active in examples layout

[julien-deramond]

Description

This PR uses the existing .bd-mode-toggle selector to force-switch the display value of the icon when active directly in the style of the layout of the examples. It's done based on the same principle as the following in site/assets/_navbar.scss:

.bd-navbar .dropdown-menu .active .bi {
  display: block !important;
}

I'm not a big fan of the current solution (even for the navbar) but this fix stays in the same spirit.

New rendering

Type of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-39177--twbs-bootstrap.netlify.app/docs/5.3/examples/features/ (for instance)

Related issues

Closes #39175