Dark/Light mode fixes (#148)

Fix for making text work with both Dark/Light mode.

Also solves line chart background to appear white in dark mode

Sources/SwiftUICharts/Base/Label/ChartLabel.swift

@@ -49,13 +49,13 @@ public struct ChartLabel: View {
     private var labelColor: Color {
         switch labelType {
         case .title:
-            return .black
+            return Color(UIColor.label)
         case .legend:
-            return .gray
+            return Color(UIColor.secondaryLabel)
         case .subTitle:
-            return .black
+            return Color(UIColor.label)
         case .largeTitle:
-            return .black
+            return Color(UIColor.label)
         case .custom(_, _, let color):
             return color
         }

Sources/SwiftUICharts/Charts/LineChart/Line.swift

@@ -94,7 +94,7 @@ extension Line {
             .fill(LinearGradient(gradient: Gradient(colors: [
                                                         style.foregroundColor.first?.startColor ?? .white,
                                                         style.foregroundColor.first?.endColor ?? .white,
-                                                        .white]),
+                                                        .clear]),
                                  startPoint: .bottom,
                                  endPoint: .top))
             .rotationEffect(.degrees(180), anchor: .center)