Fix regression: non-working direct colors in syntax files (#2252)

After 9ad4437, directly specifying color names (instead of syntax groups)
in syntax files no longer works. In particular *.patch and *.diff files
are not highlighted, since in patch.yaml direct colors names are used.

Restore the previous behavior of GetColor (fallback to direct colors if
no syntax group found) to fix this regression, but also make some changes
in StringToStyle and StringToColor to still fix the issue which was fixed
by 9ad4437. In other words, ensure that there is no confusion between
direct colors ("red", "green" etc) and syntax groups omitted in the
colorscheme file.

internal/config/colorscheme.go

@@ -35,6 +35,8 @@ func GetColor(color string) tcell.Style {
 		}
 	} else if style, ok := Colorscheme[color]; ok {
 		st = style
+	} else {
+		st = StringToStyle(color)
 	}
 
 	return st
@@ -129,15 +131,22 @@ func StringToStyle(str string) tcell.Style {
 	bg = strings.TrimSpace(bg)
 
 	var fgColor, bgColor tcell.Color
+	var ok bool
 	if fg == "" || fg == "default" {
 		fgColor, _, _ = DefStyle.Decompose()
 	} else {
-		fgColor = StringToColor(fg)
+		fgColor, ok = StringToColor(fg)
+		if !ok {
+			fgColor, _, _ = DefStyle.Decompose()
+		}
 	}
 	if bg == "" || bg == "default" {
 		_, bgColor, _ = DefStyle.Decompose()
 	} else {
-		bgColor = StringToColor(bg)
+		bgColor, ok = StringToColor(bg)
+		if !ok {
+			_, bgColor, _ = DefStyle.Decompose()
+		}
 	}
 
 	style := DefStyle.Foreground(fgColor).Background(bgColor)
@@ -158,49 +167,52 @@ func StringToStyle(str string) tcell.Style {
 
 // StringToColor returns a tcell color from a string representation of a color
 // We accept either bright... or light... to mean the brighter version of a color
-func StringToColor(str string) tcell.Color {
+func StringToColor(str string) (tcell.Color, bool) {
 	switch str {
 	case "black":
-		return tcell.ColorBlack
+		return tcell.ColorBlack, true
 	case "red":
-		return tcell.ColorMaroon
+		return tcell.ColorMaroon, true
 	case "green":
-		return tcell.ColorGreen
+		return tcell.ColorGreen, true
 	case "yellow":
-		return tcell.ColorOlive
+		return tcell.ColorOlive, true
 	case "blue":
-		return tcell.ColorNavy
+		return tcell.ColorNavy, true
 	case "magenta":
-		return tcell.ColorPurple
+		return tcell.ColorPurple, true
 	case "cyan":
-		return tcell.ColorTeal
+		return tcell.ColorTeal, true
 	case "white":
-		return tcell.ColorSilver
+		return tcell.ColorSilver, true
 	case "brightblack", "lightblack":
-		return tcell.ColorGray
+		return tcell.ColorGray, true
 	case "brightred", "lightred":
-		return tcell.ColorRed
+		return tcell.ColorRed, true
 	case "brightgreen", "lightgreen":
-		return tcell.ColorLime
+		return tcell.ColorLime, true
 	case "brightyellow", "lightyellow":
-		return tcell.ColorYellow
+		return tcell.ColorYellow, true
 	case "brightblue", "lightblue":
-		return tcell.ColorBlue
+		return tcell.ColorBlue, true
 	case "brightmagenta", "lightmagenta":
-		return tcell.ColorFuchsia
+		return tcell.ColorFuchsia, true
 	case "brightcyan", "lightcyan":
-		return tcell.ColorAqua
+		return tcell.ColorAqua, true
 	case "brightwhite", "lightwhite":
-		return tcell.ColorWhite
+		return tcell.ColorWhite, true
 	case "default":
-		return tcell.ColorDefault
+		return tcell.ColorDefault, true
 	default:
 		// Check if this is a 256 color
 		if num, err := strconv.Atoi(str); err == nil {
-			return GetColor256(num)
+			return GetColor256(num), true
 		}
-		// Probably a truecolor hex value
+		// Check if this is a truecolor hex value
-		return tcell.GetColor(str)
+		if len(str) == 7 && str[0] == '#' {
+			return tcell.GetColor(str), true
+		}
+		return tcell.ColorDefault, false
 	}
 }