some tweaks to ui theming for Fyne 2.3

ui/widgets/listheader.go

@@ -42,7 +42,7 @@ func NewListHeader(cols []ListColumn, layout *layouts.ColumnsLayout) *ListHeader
 	for i, _ := range l.columnVisible {
 		l.columnVisible[i] = true
 	}
-	l.container = container.NewMax(canvas.NewRectangle(theme.ButtonColor()), l.columnsContainer)
+	l.container = container.NewMax(canvas.NewRectangle(theme.BackgroundColor()), l.columnsContainer)
 	l.ExtendBaseWidget(l)
 	l.buildColumns()
 	return l

ui/widgets/nowplayingcard.go

@@ -36,7 +36,7 @@ func NewNowPlayingCard() *NowPlayingCard {
 	n.cover.SetMinSize(fyne.NewSize(85, 85))
 	n.cover.FillMode = canvas.ImageFillContain
 
-	n.c = container.NewBorder(nil, nil, container.New(&layouts.CenterPadLayout{PadLeftRight: 3, PadTopBottom: 5}, n.cover), nil,
+	n.c = container.NewBorder(nil, nil, n.cover, nil,
 		container.New(&layouts.VboxCustomPadding{ExtraPad: -12}, n.trackName, n.artistName, n.albumName))
 	return n
 }