setStyleSheet was overriding default tooltip theme from the system,
causing themes like Breeze to render white text on a grey bg for
tooltips. Tooltips now render default for the system theme everywhere.

set a theme matching color on the dark theme comboboxes.

fixes #118

markdown -> Markdown

May make scrolling more laggy or jumpy.

This currently assumes the event, that is replied to, is already
fetched. If it isn't, it will render an empty reply. In the future we
should fetch replies before rendering them.