Add 'alt-bg' color for striped lines

Test cases:

1. 'jump' should show alternating background colors even when 'alt-bg' is
not defined as before.

  go run main.go --bind load:jump

Two differences:
  * The alternating lines will not be in bold (was a bug)
  * The marker column will not be rendered with alternating background color

2. Use alternating background color when 'alt-bg' is set

  go run main.go --color bg:238,alt-bg:237
  go run main.go --color bg:238,alt-bg:237 --highlight-line

3. 'selected-bg' should take precedence

  go run main.go --color bg:238,alt-bg:237,selected-bg:232 \
                 --highlight-line --multi --bind 'load:select+up+select+up'

---

Close #4354

src/terminal.go

@@ -2791,17 +2791,26 @@ func (t *Terminal) printItem(result Result, line int, maxLine int, index int, cu
 	_, selected := t.selected[item.Index()]
 	label := ""
 	extraWidth := 0
+	alt := false
+	altBg := t.theme.AltBg
+	selectedBg := selected && t.theme.SelectedBg != t.theme.ListBg
 	if t.jumping != jumpDisabled {
+		if !altBg.IsColorDefined() {
+			altBg = t.theme.DarkBg
+		}
 		if index < len(t.jumpLabels) {
 			// Striped
-			current = index%2 == 0
+			alt = index%2 == 0
 			label = t.jumpLabels[index:index+1] + strings.Repeat(" ", util.Max(0, t.pointerLen-1))
 			if t.pointerLen == 0 {
 				extraWidth = 1
 			}
 		}
-	} else if current {
-		label = t.pointer
+	} else {
+		if current {
+			label = t.pointer
+		}
+		alt = !selectedBg && altBg.IsColorDefined() && index%2 == 0
 	}
 
 	// Avoid unnecessary redraw
@@ -2828,10 +2837,12 @@ func (t *Terminal) printItem(result Result, line int, maxLine int, index int, cu
 	maxWidth := t.window.Width() - (t.pointerLen + t.markerLen + 1)
 	postTask := func(lineNum int, width int, wrapped bool, forceRedraw bool) {
 		width += extraWidth
-		if (current || selected) && t.highlightLine {
+		if (current || selected || alt) && t.highlightLine {
 			color := tui.ColSelected
 			if current {
 				color = tui.ColCurrent
+			} else if alt {
+				color = color.WithBg(altBg)
 			}
 			fillSpaces := maxWidth - width
 			if wrapped {
@@ -2899,7 +2910,7 @@ func (t *Terminal) printItem(result Result, line int, maxLine int, index int, cu
 			}
 			return indentSize
 		}
-		finalLineNum = t.printHighlighted(result, tui.ColCurrent, tui.ColCurrentMatch, true, true, line, maxLine, forceRedraw, preTask, postTask)
+		finalLineNum = t.printHighlighted(result, tui.ColCurrent, tui.ColCurrentMatch, current, true, line, maxLine, forceRedraw, preTask, postTask)
 	} else {
 		preTask := func(marker markerClass) int {
 			w := t.window.Width() - t.pointerLen
@@ -2929,6 +2940,10 @@ func (t *Terminal) printItem(result Result, line int, maxLine int, index int, cu
 			base = tui.ColNormal
 			match = tui.ColMatch
 		}
+		if alt {
+			base = base.WithBg(altBg)
+			match = match.WithBg(altBg)
+		}
 		finalLineNum = t.printHighlighted(result, base, match, false, true, line, maxLine, forceRedraw, preTask, postTask)
 	}
 	for i := 0; i < t.gap && finalLineNum < maxLine; i++ {