message: Fix dark mode assert with wxGTK 3.0
  wx 3.0 didn't support alpha channel in HTML exports, so just tweak the
  dark mode color to not require it. It's not worth doing complicated
  fixes for the old wx version.
  Fixes
diff: @@ -106,7 +106,14 @@ wxColour ColorScheme::DoGet(Color color, Mode mode)
  case Color::ItemContextFg:
  return mode == Dark ? sRGB(180, 222, 254) : sRGB(70, 109, 137);
  case Color::ItemContextBg:
  - return mode == Dark ? sRGB(67, 94, 147, 0.6) : sRGB(217, 232, 242);
  + if (mode == Dark)
  + #if wxCHECK_VERSION(3,1,0)
  + return sRGB(67, 94, 147, 0.6);
  + #else // see https://github.com/vslavik/poedit/issues/524
  + return sRGB(67, 94, 147);
  + #endif
  + else
  + return sRGB(217, 232, 242);
  case Color::ItemContextBgHighlighted:
  #if defined(__WXMSW__)
  return sRGB(255, 255, 255, 0.50);
  