Issue 634 patch: updates for bootstrap dark mode support

[hho6643]

src/scss/tom-select.bootstrap5.scss set $select-color-text and $select-color-item-active-text to '$input-color' as text, rather than hard-coded hex values tweak $select-color-item so that it is OK both in light & dark mode define $select-color-dropdown-item-create-text as rgba($input-color, 50%) to support variable color name

src/scss/_items.scss remove SASS functions for calculating offset colors, since we now may have a variable rather than a hex number.

src/plugins/dropdown_header/plugin.scss switch hover to using a CSS filter (200% contrast), rather than SASS darken function, since we may now be passing a variable rather than a hex value

doc_src/pages/examples/styling.njk add in example for Bootstrap 5 dark mode

doc_src/js/index.js update style picker to allow for Bootstrap 5 dark mode

doc_src/css/stylesheet.scss add a min-width & auto-wide on the theme select box, it's shrinking to weird sizes without this

[github-actions[bot]]

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