I think better to put this style under .smalltalk button

But then people will still have to override it to specify the font-family they want buttons to normally have, so that the buttons will by default match their global styling. Better to fall back to the application's fonts, to ensure compatibility with any i18n/a11y issues for fonts. (A library should really not be setting font families in required CSS, unless it's to use a specialized font like an icon library -- everyone will just end up having to override it with more-specific selectors, adding unnecessary overhead.)

In this case, would be great to check styles of Cloud Commander and make changes if needed, so everything worked as it was before modification.

Anyways you can always use !important keyword.