[Feature Request] Add a role='button' attribute to the theme selector drop down

[ehakram]

Is your feature request related to a problem? Please describe. When running docfx generated websites through PageSpeed Insights, I get the following: "[aria-] attributes do not match their roles." For more context: https://dequeuniversity.com/rules/axe/4.7/aria-allowed-attr

Describe the solution you'd like Probably: replace line 40 from docfx/templates/modern/src/theme.ts with the following: Notice that role='button' was added.

Thank you.