Set navigation bar color to dark on dark theme (#3091)

* Allow overriding ConversationsTheme.Dark

This change splits the dark theme style into two styles - base that
stays the same in all configurations and Dark - the original - that can
be overridden in specific configurations to add version specific styles.

* Set navigation bar color to dark on dark theme

Use secondary background color as navigation bar color when using dark
theme. This avoids mismatched white colors on certain system themes.