[splittiming] No BG Changes in Step Timing.

Themes/_fallback/Languages/en.ini

@@ -1258,6 +1258,7 @@ This change creates notes past the end of the music and is not allowed.=This cha
 This will destroy all unsaved changes.=This will destroy all unsaved changes.
 Undo=Undo
 You must be in Step Timing Mode to edit the notes.=You must be in Step Timing Mode to edit the notes.
+You must be in Song Timing Mode to edit BG Changes.=You must be in Song Timing Mode to edit BG Changes.
 
 Current beat=Current beat
 Current second=Current second

src/ScreenEdit.cpp

@@ -1279,6 +1279,8 @@ static int FindAttackAtTime( const AttackArray& attacks, float fStartTime )
 }
 
 static LocalizedString TAP_NOTE_SONG_TIMING	( "ScreenEdit", "You must be in Step Timing Mode to edit the notes." );
+static LocalizedString BG_CHANGE_STEP_TIMING	( "ScreenEdit", "You must be in Song Timing Mode to edit BG Changes." );
+
 static LocalizedString SWITCHED_TO		( "ScreenEdit", "Switched to" );
 static LocalizedString NO_BACKGROUNDS_AVAILABLE	( "ScreenEdit", "No backgrounds available" );
 static ThemeMetric<bool> INVERT_SCROLL_BUTTONS	( "ScreenEdit", "InvertScrollSpeedButtons" );
@@ -1925,6 +1927,10 @@ void ScreenEdit::InputEdit( const InputEventPlus &input, EditButton EditB )
 				EditMiniMenu( &g_BackgroundChange, SM_BackFromBGChange );
 			}
 		}
+		else
+		{
+			SCREENMAN->SystemMessage( BG_CHANGE_STEP_TIMING );
+		}
 		break;
 
 	case EDIT_BUTTON_OPEN_COURSE_MENU: