UX: Ensure external login icons are visible on hover (PR #13914)

Some authentication buttons (e.g. apple, oidc, oauth2, saml) do not have a specific color specified. Therefore they were taking the default button-with-icon color, and the icons would almost disappear on hover. This commit adds a default of #000 for these buttons, so that the button hover looks similar to core auth buttons.

Hover state before:

Screenshot 2021-08-02 at 13 43 51

Hover state after:

Screenshot 2021-08-02 at 13 43 36