message: Tweak FileViewer dark mode appearance
diff: @@ -103,10 +103,11 @@ FileViewer::FileViewer(wxWindow*)
  ColorScheme::SetupWindowColors(this, [=]
  {
  + // match CSS background color:
  if (ColorScheme::GetWindowMode(this) == ColorScheme::Light)
  SetBackgroundColour(*wxWHITE);
  else
  - SetBackgroundColour(GetDefaultAttributes().colBg);
  + SetBackgroundColour("#1d1f21");
  });
  wxPanel *panel = new wxPanel(this, -1);
  @@ -750,6 +751,10 @@ mark {
  padding-top: 1em;
  }
  +.message h2 {
  + font-weight: 600;
  +}
  +
  .explanation {
  width: 80%;
  margin: 1em auto;
  