CSS for multi-selects uses opacity var, single-selects doesn't

Describe the bug For single select, the opacity rule for #{$choices_selector}__button is of a fixed value - no variables to override. This seems like a mistake, because multi-selects provide variables for that opacity value.

To Reproduce See choices.scss on lines 113 and 175. The former does not utilise any CSS or SCSS vars.

Steps to reproduce the behavior: See above.

Expected behavior If CSS and SCSS vars are defined for the opacity of multi select buttons, they should probably also be used for single select (select-one) buttons.

Screenshots Not applicable in this case.

Choices version and bundle

• Version: v11.2.3

Desktop (please complete the following information):

• OS: macOS
• Browser: Chrome
• Version - OS: 16.7.4, Chrome: 150

Smartphone (please complete the following information): Not applicable in this case.

Additional context I am happy to PR this. The fix is a one-liner:

opacity: var(--choices-button-opacity, #{$choices-button-opacity});