Add theme properties for empty state indicator text

[megantaylor]

Motivation

Want to customize styling of text rendered by the EmptyStateIndicator component

Proposed solution

Add theme properties for the test rendered by the EmptyStateIndicator component

there are existing properties applied to the text rendered for an empty channel, but not for the "message" case or the "default" fallback case

Acceptance Criteria

developer can set styles in the theme that apply to all text elements in the EmptyStateIndicator component