Add theme support

Fix some minor bugs
Add default and helloworld themes
Update README

theme.go

@@ -0,0 +1,61 @@
+package termui
+
+type colorScheme struct {
+	BodyBg            Attribute
+	BlockBg           Attribute
+	HasBorder         bool
+	BorderFg          Attribute
+	BorderBg          Attribute
+	BorderLabelTextFg Attribute
+	BorderLabelTextBg Attribute
+	ParTextFg         Attribute
+	ParTextBg         Attribute
+	SparklineLine     Attribute
+	SparklineTitle    Attribute
+	GaugeBar          Attribute
+	GaugePercent      Attribute
+	LineChartLine     Attribute
+	LineChartAxes     Attribute
+	ListItemFg        Attribute
+	ListItemBg        Attribute
+	BarChartBar       Attribute
+	BarChartText      Attribute
+	BarChartNum       Attribute
+}
+
+// default color scheme depends on the user's terminal setting.
+var themeDefault = colorScheme{HasBorder: true}
+
+var themeHelloWorld = colorScheme{
+	BodyBg:            ColorBlack,
+	BlockBg:           ColorBlack,
+	HasBorder:         true,
+	BorderFg:          ColorWhite,
+	BorderBg:          ColorBlack,
+	BorderLabelTextBg: ColorBlack,
+	BorderLabelTextFg: ColorGreen,
+	ParTextBg:         ColorBlack,
+	ParTextFg:         ColorWhite,
+	SparklineLine:     ColorMagenta,
+	SparklineTitle:    ColorWhite,
+	GaugeBar:          ColorRed,
+	GaugePercent:      ColorWhite,
+	LineChartLine:     ColorYellow | AttrBold,
+	LineChartAxes:     ColorWhite,
+	ListItemBg:        ColorBlack,
+	ListItemFg:        ColorYellow,
+	BarChartBar:       ColorRed,
+	BarChartNum:       ColorWhite,
+	BarChartText:      ColorCyan,
+}
+
+var theme = themeDefault // global dep
+
+func UseTheme(th string) {
+	switch th {
+	case "helloworld":
+		theme = themeHelloWorld
+	default:
+		theme = themeDefault
+	}
+}