message: Fix sidebar hover highlighting in dark mode
  Ligthen the background instead of darkening it.
diff: @@ -298,7 +298,9 @@ public:
  // setup mouse hover highlighting:
  m_bg = parent->GetBackgroundColour();
  - m_bgHighlight = m_bg.ChangeLightness(95);
  + m_bgHighlight = ColorScheme::GetWindowMode(parent) == ColorScheme::Dark
  + ? m_bg.ChangeLightness(110)
  + : m_bg.ChangeLightness(95);
  wxWindow* parts [] = { this, m_icon, m_text, m_info, m_moreActions };
  for (auto w : parts)
  